Spirit

Spirits are a large class of incorporeal lifeforms, many originating from separate worlds known as Spirit Realms. They tend to possess powers and abilities that exceed the boundaries and logic of the physical plane of existence. But while their biology and possibly morality is very alien, many species have adapted to the natural setting of the world in various ways.

Classification of spirit attributes 

Physiological Abilities: possessed by all spirits Common Abilities: possessed by most spirits Uncommon Abilities: possessed only by specific class of spirits Rare Abilities: possessed only by a few spirits

Physiological Abilities

Spirits are composed of a class of substances known as subtle matter that usually does not interact with normal matter, so they function differently than most corporeal beings and that means corporeal lifeforms cannot usually feel or sense the presence of spirits around them. Subtle matter instead largely interacts with Psychokinetic Energy which forms the basis of many spiritual powers and abilities. 

Intangibility: All spirits can induce the ability to pass through solid objects and many forms of energy.
Invisibility: All spirits can induce the ability to render one's body undetectable to the naked eye if they aren't already.
ESP: Due to the nature of being composed of subtle matter, spirits are unable to use any of the physical senses to perceive the world when incorporeal, so no touch, smell, sight, etc. Instead they normally perceive the world through a set of senses known as extrasensory perception (ESP). Spirits produce and depend on psychic energy in the same way that organic beings produce electromagnetic currents and fields. Spirit physiology produces psychic waves that enable them to respond to stimuli in their ethereal environment, move themselves or things around them, navigate the world, and react to psychic energy outside of them.
Manifestation: Spirits have developed various means of creating a form in the material plane that can interact with the everyday physical world. 
Metaphysical Sustenance: Spirits can get energy and structural nutrition from ambient subtle matter, mana, or psychic energy in their environment. 

Common Abilities

Material Food Independence: Since spirits have a metaphysical diet, this means most don't need to eat or drink foods that physical beings do although many can make bodies that are capable of doing so if they wish to. The main exception being those spirits whose physical forms require structural nutrition from solid or liquid materials. While spirits usually do not require this type of sustenance in the first place, they also produce bodily waste after consuming food if they have a digestive system.
Flight: Flight is the ability of spirits to hover and move in the air at will by floating on psychokinetic energy.
Agelessness: Most spirits do not deteriorate or die from the aging process. Any that can have a strong affinity to the physical plane and can live for long periods of time. However, most can still be killed from injury or die of starvation. Sleeplessness: Spirits do not require rest or sleep to normally function as long as they work at a moderate pace. They are however capable of becoming tired from overworking, and if a spirit exhausts themselves, sleep can restore their stamina faster.
Breathlessness: Most spirits do not need to breathe to stay alive or function properly although they are physically capable of doing so in a physical form.
Ectoplasm Production: Ectoplasm is another type of subtle matter spirits can produce. It acts as an intermediary substance since unlike teleplasm it can also phase in and out of tangibility and affect physical objects.
Telekinesis: Spirits have the power to move objects without being in physical contact with them, by using psychokinetic energy.

Uncommon Abilities

Teleportation: The ability to move instantaneously from one location to another.
Transformation: Spirits can morph (usually for a limited time) into alternate versions of their default state. At times, the appearance of such forms is merely cosmetic, while in others, the user's abilities are completely altered in order to tap into greater stores of energy, strength, speed and magic. These states can either be willfully invoked or only occur under specific conditions like a certain state of mind.
Shapeshifting: Some spirits have the ability to change the physical appearance of their manifested physical body to that of another living or nonliving object.

Rare Abilities

Specific Manipulation: Certain spirits can manipulate various forms of matter and energy, some specific to kind and includes but not limited to fire, water, earth, air, electricity, light, plants, etc. 
Transformation: Spirits can morph (usually for a limited time) into alternate versions of their default state. At times, the appearance of such forms is merely cosmetic, while in others, the user's abilities are completely altered in order to tap into greater stores of energy, strength, speed and magic. These states can either be willfully invoked or only occur under specific conditions like a certain state of mind.

Manifestation

While the physical form a spirit can choose could be virtually limitless, there are definitely some recurring shapes. The reason for it is unknown but some speculate that spirits are somehow memorizing certain forms from the collective unconsciousness that is propagated in the spiritual plane. These are the most common ways for spirits to undergo manifestation.
Direct
Some spirits can directly manifest a physical body to interact with the physical plane. A spirit will expend magical power (either generated or absorbed around them) to collect or conjure their bodies' components and infuse their vitality into it. Magic power is needed to maintain their physical bodies integrity, either by producing it themselves or absorbing from the environment.

The manifestation is a corporeal living construct, thus it is affected by normal matter and now intangible to the constructs of the spiritual plane. As it was created and sustained by essentially the spirit's life force, all sensations of the physical world will be experienced by the spirit through the construct. Even in this form, spirits generally do not need to breathe, eat, excrete or sleep as they are homogeneous constructs that lack organ systems by default and gain their necessary energy in alternate ways. Of course they have the option to create internal organs in their bodies so that they can perform these functions if they wish.
A spirit who manifests as a live construct is typically capable of full telepathic control of the body even dissipating and reforming it at will. This grants the body a form of intangibility as they can let most physical objects pass through the body or move it out of the way. The exception to this is if they are struck by specific magical objects or psychic attacks. Such attacks can cripple or outright slay a manifested spirit's body, forcing it to release its physical form, reduced to a weakened wisp of their incorporeal form and some can even outright kill a spirit. This wisp is basically the remains of a spirit and will use the last of its power to inhabit an object they come across if they have enough affinity with it. When this happens, the spirit must reside in this object for an extended period of time until it can recover its strength and reform.
Possession
All spirits in an incorporeal state can enter and operate preexisting objects or creatures to interact with the physical world to a certain degree. However the exact kind of object or creature a specific spirit has the most affinity with and thus have the best control depends on its species. Spirits possessing a living creature are subject to the physical needs of that creature like food. Unlike direct manifestation, the connection is more unstable and a living creature can mentally fight the spirit out of its body through will.
The physical body can be attacked or killed, but the spirit can simply leave the body at any time, leaving the hapless victim to suffer whatever fate was meant for the entity. To harm the ethereal being requires metaphysical means such as magic, psi or a weapon meant to harm spirits and being persistent once it leaves the body.
Avatar Method/Incarnation
The third option comes in two flavors: the first involves spirits possessing physical living beings, either very young, or unborn. If possessing a infant creature, the full incarnation takes longer, but if pre-born, it's instant. This method results in the spirit effectively fusing with the creature's consciousness and experiencing physical life from the beginning to its end. Avatars typically regain their memories as spirits later in their lives.
The second flavor explains the nature of beings that are born as hybrids of corporeal beings and spirits. They typically come from the union of an organism and a spirit that formed reproductive organs in their constructs to reproduce.
Unlike most spirits that rely mostly on psychic force to live or magic power to stay physical, avatars only rely on chemical nutrients to maintain their bodies and so they have to breathe, eat, drink food for life-sustaining metabolism. And if they are a species that sleep, they will require rest as well to properly function.
Spirits who have taken or born as avatars are vulnerable to physical harm and anything that can harm a corporeal being can harm the spirit. These spirits do not use psychic energy to regenerate like others, and must either spend other resources like magic to heal, or heal at a natural rate. Metaphysical weapons and faith based rituals will harm them far less than other spirits though, because they are more attuned the physical world.

Broad Spirit Types

Jinn: Jinn are spirits that are native to the Aether , the parallel spirit realm of Kaf that connects all others to the material world. They are composed of electrically charged ectoplasm known as smokeless fire. They can physically manifest by building electrostatic charges to ionize the air into plasma that collects ambient dust to their ectoplasm and make a physical construct.


Divine: Divine spirits or divinities are composed sacred teleplasm which can produce of a form of ectoplasm known as ichor, which can directly manifest as light in the physical realm or solidify into photonic molecules with mass. Another signature of divinities is theurgy, their exclusive form of sorcery that allows them to influence, manage or guard a natural, preternatural, or supernatural aspect of reality.
For a large part of Kaf's history, divinities were the object of worship and various belief systems and rituals created to appease them and encourage them to keep the world running and maybe grant fortune on some. In return, divine spirits did much in favor of mortal beings that provided them with praise and even empowered beings into heroes who did great deeds to inspire others to worship. Divinities were behind the creation of the multiple spirit realms for their dwellings, using the Astral Plane as an anchor point.
By the time the Ghulat Wars began, divinities all over had their followers fighting for them until the Shaytan and demons brought the fight to them. A lot of spirit realms were destroyed and their were many causalities on both sides. To atone for the damage to the world caused by the wars, most divinities stepped back from their management role with the major ones deciding to enter a cycle of reincarnation as various mortal species while others are scattered throughout the world and remaining spirit realms.

Demonic: Spirits that originate from unclean teleplasm or were corrupted by Demonic Ectoplasm. Both substances have psychic energy and emanations supersaturated with negative emotions and impulses with can corrode the psyche of other lifeforms, either killing them or turning them into beings known as fiends. Their ectoplasm will then assimilate the molecules of other physical beings to exist in a physical form.
Because of the energies involved in their creation, the vast majority have aggressive, and cruel tendencies though there have been individuals that are neutral or even benevolent.
They might be either malevolent, neutral, or maybe even benevolent in their dealings with other creatures, but their reputation in general is dire and grim and they are widely seen as cruel, violent, and fiercely chaotic.
Demons weren't on the scene until the Ghulat Wars, conventional wisdom and records implying that the Shaytan created the demonic strain of energy. Demons mainly sided with the invasion force of the Shaytan or served themselves. The majority were hostile to life in general, especially Kaf natives. In the aftermath of the warrior, nearly all demons were vanquished by heroes or the natural disasters that ravaged the world after, though there are traces of demons and rumors of a few realms they were banished too.

Nature: Nature spirits are those attracted to and have an affinity for the nature of the physical realm, whether it be matter, energy, life, etc. When first formed, they exist as featureless animist teleplasm wisps simply wandering the spiritual realm of the planet for energy and barely conscious of the physical plane. At some point in their lives, they will suddenly become very aware of the world which triggers an urge to form a physical body. These bodies are typically formed of a unique combination of their ectoplasm and the elements of the natural physical environment and the many seem to assume forms reminiscent of either plants or animals, often humanoid ones.

Undead: Undead spirits are those created after the death of a living creature. Undead spirits normally arise from necromagy spells and rituals, generally copying the consciousness of the deceased into a mass of inert ectoplasm to animate, in a sort of resurrection like manner. The psychic energy animating these spirits is often negative in nature, typically resulting in aggressive or melancholic personalities, many cases having animosity towards living beings.