The Art of Worldbuilding 1: Creating Life

Gods

Typically a fantasy world will have deities be real entities, which could be universal creators or merely powerful pseudo-immortal figures. Gods will predate their religion, and many different groups may worship them in different ways, creating different religions. Gods can be universal, or worshiped by only a select few (people or races). It is often better to have a few general gods in common between the races, and then a few specific gods for each race / culture.

Pantheons

A pantheon is a collection of gods grouped and worshiped together. These gods will have relationships with each other, and may represent opposing or conflict ideas and beliefs within a culture. A pantheon can have a culturally formal structure and/or a metaphysical structure, depending on the origins of the individual deities.

Gods are not all created equal. If the pantheon is a family, the divine parents may be more powerful than their children. Alternatively, gods may grow and shrink in power based on the number of worshipers. Gods are often capable of reproducing with mortals to some degree, and these entities may represent lesser domains or have limited worship. Lesser / demigods may also have once been mortals which ascended to godhood. These power differentials can be the source of conflicts in myths.
Gods are typically invulnerable and immortal from a human perspective. But if gods can be born or created, it is just as possible for them to die. This can create different eras of time for the birth and death of different deities. It is likely the dead gods were destroyed by something specific, inviting the creation of powerful magics which are strong enough to hurt them. If their strength is tied to their worship, then the races or cultures with religions for the god are directly tied to their power and possible mortality.

Myths

Different gods and collections of gods will have stories told about them, called myths. These myths can be true or false, and could be stories the gods told the races or ones the people made up / discovered for themselves. The myths may have many layers of truths which have been obscured over time. Societies are almost guaranteed to have myths pertaining to: creation, origin of life and celestial bodies, death and afterlife, and the end of the world. Other common myths involve the all of the activities performed by humans being performed by gods, from falling in love, the becoming parents, jealousy, loss, invention, etc. Gods will also often have magical artifacts associated with them, whose powers are often detailed in myth.

Characteristics

Gods can fit into the same alignment structure that other characters do. This could include distinctions of good vs. evil. However, most gods, just like most people won't consider themselves evil and will have a rationalization for their actions and beliefs.
Gods will also have a set of identifiers beyond merely their name. This includes titles (God of War, Lord of Despair), the types of people they patron (farmers, warriors), and common symbols of their power and domains. If the gods can manifest in the physical world, to they likely have a standard set of appearances. They can also be associated with an element, a color, or even a season.

Behavior

Each god will have a personality and behavior pattern. Depending on their position in the pantheon, this could prompt good or poor relationships with the other gods, and may result in some type of metaphysical punishment. The behavior of the deities will likely also reflect the types of the behavior they expect from their followers, and what kind of punishments are administered for failure. The common behaviors of the god and their followers will also develop a reputation for them among the other inhabitants of the world.

The behavior of the deity will also manifest in how the interact with the world. Often their will be some kind of restriction preventing the god from interacting or interfering directly in worldly affairs. They may have to be summoned or prayed to in order to appear. Their behavior also manifests in the types of creatures and places that they create and occupy, as well as the places which are created to serve their worship.

Species

There is an important distinction to make between species, races, and ethnicity. For everything found in this world, different species cannot bread with each other and produce offspring, but different races and ethnicities can. Humans and elves are the same species, but different races within that species. Dark and Wood elves are the same race but different ethnicity. Race is roughly equivalent to sub-species.

Habitat

On Earth, with no competition for humans, it is easy to forget that each species/race should be shaped by their environment. If their are multiple species, each group is likely to be optimized for the specific habitat it lives in, having advantages over an invading race but disadvantages when invading itself. Worth considering is how nomadic and adaptable the race is, which will affect its distribution in the world. If they do move around, consider the history of why, as habitat and history will directly affect each other.

Isolated Settlements

An isolated settlement is one only or predominately inhabited by members of the founding race. As such it should be expected to be optimized to the needs and desires of that race. Consider the natural resources available in the area, as well as the technologies known to the people and how they use them. Also consider why they are isolated. It could be a universal truth of the race, or it could be a cultural difference brought on by historic factors. In the second case different settlements by the race are likely to have varying levels of integration.

Joint Settlements

More likely than isolation is that as a settlement grows it will attract visitors and traders from different cultures and races, some oh whom will settle there as well. This will gradually produce a melting pot as two or more groups have to work out how to live and work in constant close proximity. This can be an interesting source of current tension and can be fuel for compelling historic events.

Terrain and Climate

The terrain should affect and inform the adaptations of the species that lives in it. Similarly different species will have different tolerance levels for different climates. The biggest distinction will be between marine and terrestrial, and then between above ground and below ground for the terrestrial. But even species that live above ground, there is a huge difference in needs and problems between forests, plains, and mountains.

Disposition

Is the race generally friendly and benevolent, or generally aggressive and cruel? Real humans are vary complicated and have a great deal of variety, so how much variation is there within the new species? If they are fairly identical in disposition, is it because a god created them that way? How do other cultures, races, etc. perceive the group, and how does that clash with their own perception of themselves?

Appearance

The appearance of the races helps define expectations about them. It will influence not only their suitable habitats but also their culture and societal expectations. It is important to distinguish between humanoids and non-humanoids. Humanoids have the advantage of being easier to understand and easier to slot into existing human societies and technologies, but non-humanoids can be more distinct and creative. Remember that each body feature added should have an adaptation use that benefits the species in some way, which could be vestigial depending on changing circumstances. Also consider how uniform the group is, both in its perception by outsiders and internally. If the species is humanoid (or even if not) do they were clothes, and what do those clothes say about their culture? There could be types of clothing or variations which are specific to that particular group. This also extends to accessories such as jewelry, weapons, and armor.

Characteristics and World View

One way to help define a species is to use the ability scores of a game system and plot how this species compares to the average in the world. However it's important to note that they should not be designed as "humans but different in some way." This can be especially hard when defining their worldview, because most humans are actually remarkably similar in that regard. It is good to question stereotypical human reactions and behaviors, and think about what the root of those behaviors are and how they would be different with different factors affecting them.
An excellent place to include this variety is in the structure of the society. Any sufficiently large group will need some kind of government to keep the peace between members, which broadly fall into republic, monarchy, or dictatorship. What are the common social institutions of the group, such as marriage, divorce, and religion? How large is the typical family unit and how long does it last? Similarly how big is the larger community surrounding the family? How sophisticated are the laws of the society? This will depend on the lifespan of individuals and the size of the community, as well as the types of conflicts which typically arise.
How does the species communicate? Do they have a written and/or spoken language? How is that language constructed? Consider the regional dialects and differences within the species in disparate geographies. Alternatively, is there a common language which is shared or spoken by many cultures or groups? Consider also the common customs of the people, such as greetings and farewells, expectations and taboos. These can be an interesting if cliche source of conflict between groups.
Consider also the history of the species. Some of this will develop over time, but it is a good idea to know where and how the species originated, and where and how it has migrated to since then. Consider if this migration resulted in contact with preexisting groups or movement into previously unknown lands, and what kind of conflicts arise from the movement. Major events and factors include wars and history of conquest or enslavement, both internally and with other groups.

Relationships

Every group will have a set of relationships with all of the other groups. Some of this will come predefined based on the mutual attitudes and beliefs, but some can develop overtime as the two groups develop shared history. Remember to include all of the cross relationships between all of the different species and slowly develop a web of interactions.

Supernatural

If some kind of magic exists in the world, the interactions of the species with it will also need to be defined. It is best to first define how prevalent each type of supernatural event is in the world, then define the species relationship to it. The first category is "natural" phenomena such as magical terrains, weather, or materials. How well known is the phenomena, including how it acts, how dangerous it is, and its relative utility? The second category is more traditional magic, that which can be called upon by its practitioners to affect the world. This ability could be innate or learned, and may require outside components to control. Defining the magic system in the world will help determine the affects it has on each of the different species. A counter option is the have magic power granted by the divines. This could come with strings attached or specific requirements. Different groups may receive different powers from different deities, or place different relative importance on innate magic versus divine.

Technology

Technology is often the counterpart to the supernatural, in that they vary inversely with each other between fantasy and Sci-Fi worlds. It is important to define what level of tech the civilization operates at, and how that compares to nearby groups. This includes weaponry, farming tools, transportation tools, food processing tools, building tools, and more.

Plants and Animals

Plants and animals are typically easier to create than a full species because they are usually less complex. Of course, we only need to spend time creating them if they are different enough from real species to justify the work. 

Plants

The most important thing to determine about plants is the climate they live in, as this will directly affect most of their traits. 

Seedless plants include algae, liverworts, ferns, and mosses. Algae will be most important to water based cultures and nearly irrelevant outside of them. Ferns are usually simple background plants (tropical) and are typically not the subject of quests. Mosses typically grow in damp regions on almost any surface. They can come in a variety of colors, and are a good pick for weird properties necessary for magical potions.
Cycads have cone seeds in the center of a fern-like group of leaves atop a central stalk (includes palm trees). They are incredibly long lived and are found mostly in tropical and subtropical regions.
Conifers also have seed cones but are found in temperate to colder regions. Most often occur as trees, but also found as shrubs, with common members including Douglas-firs, junipers, redwoods, spruces, and pine trees.
Flowering plants dominate the temperate zone, which is the most common setting for most stories. These are flowers, shrubs, vines, and trees including oak, maple, elm, aspen, birch, and fruit trees. They come in two varieties, deciduous loose leaves in winter, evergreens sheds leaves all year round but never loose all of them. 
Plants fall into four categories of lifestyles. Trees, shrubs and vines are perennials have incredibly long lifetimes, though they only flower at certain times of year. Annuals grow, produce seed, and die of the course of a year, whereas biennials live two years, including most vegetables which produce food the first year then seeds the second. Bulbs are a type of perennial which have a central underground root bulb which persists year after year when the rest of the plant dies. 

Animals

Invertebrates: these back up 97% of animal life (one earth) and have the greatest diversity of body plans and lifestyles. They typically very small because their body size is dependent on the amount of available oxygen, but under extreme circumstances they could become huge terrifying monsters. Their most common interaction with civilized races is as pests, but a very few species can be domesticated (honey bees).

Fish and Aquatic Life: A vertebrates descend from the tetrapod body plan of some fish species. The broader category of fish also includes eels, lampreys, rays, and sharks. Aquatic life also includes all forms of whales, those these are actually mammals. Fish are cold blooded with fins and gills, typically have poor hearing with motion based vision. Several species can fly through the water for hundred of feet of breath on land for up to minutes. Schools are tightly organized colonies of fish, while shoals refer to independent fish that stay close to each other. Fish are primarily either food sources or threat sources (poisonous, toxic, or large enough to eat people). However, they come with built in symbolism as a food source, especially if only they persisted during a drought.
Amphibians: Frogs, toads, and salamanders, which require water to breed and have larva with distinct metamorphosis. They are typically carnivorous, preying exclusively on animals smaller than themselves (insects). They are also typically small, but it could be fun to have large variants. Some are either poisonous or venomous which may make them of interest to people as weapons. 
Reptiles: Turtles, crocodiles, lizards, and snakes. Cold blooded with leathery skin insufficient for armor. They are slow moving animals with slow metabolisms who thrive in regions with too little food to support birds or mammals. They are generally carnivorous and active during the day, unless they need to compete with birds/mammals then they move to nocturnal. They typically are not very intelligent, but often posses standard venom and poisons, or weird defense mechanisms like tail detachment. 
Birds: Birds have more species than mammals. Whether herbivorous or carnivorous, they always consume prey smaller than them (no teeth in their beak mouth). Can be useful pets (intelligent enough to train) and are a useful source of food in either eggs or meat. With the gift of flight, they often migrate either for food concerns or breeding needs, and have great navigation senses. They flock for defense, and are smart enough to work together with other birds and other species. Birds are typically diurnal, unless competing with better adapted species or hunting specific prey at specific times. When isolated from predators (islands) they often become flightless and massive. All birds lay eggs hidden from predators, with 10 to 80 day incubation of 1 to 12 eggs. They can theoretically become massive (couple hundred kg) but only if there is enough food to support them.
Mammals: Currently the largest and smartest animals, and the most dominant in each of their habitats. Sea mammals are an obvious draw, including those that must remain in the sea (whales) and those which are semi-aquatic (otters, seals, sea lions). Other mammals have varying degrees of aerial mobility, from the dozens of tree-dwelling species that can glide to bats which can actually fly. Some mammals have not quite mastered flight but are still adapted for trees, with longer limbs and tails or improved jumping abilities. Those that are fully terrestrial have three types of walking. Plantigrade (primates, including humans) locomotion plants the toes, metatarsal bones, and heels all on the ground. They are slower (typically worsened by shorter, thicker legs) but are more stable and can carry more weight. Digitigrade locomotion (cats and dogs) are balanced on their toes making them faster and quieter. Ungulate locomotion (ungulates) walk of the very tips of their toe(s) with a hoof toenail that grow perpetually. They are typically herbivorous and typically the fastest. All animals nurse their young and typically give live birth. They are all warm blooded, making them more active but requiring more food to maintain their metabolism, and making them susceptible to hyper or hypothermia. They are the most common animal used by civilized races, for food, leather, wool, pets. beasts of burden, and entertainment. 

Purpose

Typically the only reason we create a species is because it has a special purpose in the story or for the races interacting with it.

Plants: Plants are obviously a source of food, but they are also used in decorations, medicines , building materials, clothing, and tools. Different portions of the plant will have different utility, and some may have to be treated before they can be used because they are poisonous. Many plants have specific decorative uses, such as evergreens for winter holidays and flowers for special ceremonies. Plants not used specifically for food might still have medicinal properties when ingested due to their chemical nature. This might include supernatural or magical properties.
Animals: The animals created should be distinguished from real animals to make them feel like they are a meaningfully different creation. The most common animals a culture will interact with are those which have been domesticated, either as beasts of burden, as food, as sources of sport, pets, or guards. The teeth, bone, hide, claws, and fluids of the animal might have specific alternative uses, whether or not the animal is domesticated. 
These species are usually created either by taking existing species and tweaking traits (make three changes) or by making a list of products then creating the things which provide those products.