Orcs

Orcs are technically cousins of the goblins, whose origins are shrouded in darkness. This is caused mostly by the fact that the true history of the goblins (and their past greatness) remains unknown to the world at large nowadays.
In distant past, the orcs were to goblins what the high elves were to Xyls - an entire biological caste/species designed for nothing but war. Entire armies of orcs were raised to battle the servants of the ancient dragons, only to eventually be defeated and scattered. The orcs as a species have nearly died out, the goblins prioritizing their own while retreating into the Labyrinth.
The few small tribes of orcs managed to survive on the fringe of civilization, being on their best way towards extinction. They recovered during the Pre-Imperial Era, when the vast and neverending wars made them be seen as useful mercenaries and allowed them to multiply. This, however, often backfired on their employers, with orkish mercenaries ending up seizing control over collapsing elven countries, only to be eventually driven off by the humans.
Most of those holdouts were crushed, the orcs either wiped out or somewhat forcefully 'civilized'. This allowed them to survive past the Second Era, only for most of them to decivilize in the aftermath of the Twilight War, usually falling into the worship of either Nine Hordes or the Immutable Hierarchy). The former, despite their savage behaviour, are still treated by the Grand Empire and the local client states as useful source of the auxiliaries.

Basic Information

Anatomy

Orcs are tall (about 2,0m), almost uniformly muscular and inhumanly resilient natural-born warriors, with leathery skin and a mouth filled with fangs. Their skeletal system is significantly reinforced, although not through a number and placement of bones (with some slight modifications) but through their resistance to hits.
Unfortunately, this makes them inferior swimmers (as they tend to drown quickly as they can't float due to their bone density). Fortunately, especially their ribcage is practically a natural armor protecting the most important organs.

Biological Traits

The orcs aren't a uniform species, and in fact they never were. This has gotten objectively worse in the modern times, with certain High Gods deciding to make some modifications to their species in order to better fit their needs. The basics are the same for the all the species, with the differences more often than not being in their minds than their bodies.
Gray Orcs - The gray orcs are the default, original form of the orcs, easily recognizable by their dark gray skin. Today majority of the surviving gray orcs are counted among the civilized inhabitants of the Grand Empire, being surprisingly often seen among the adventurers and welcomed among the mercenary groups and even the Imperial Army's auxiliaries.
Green Orcs - The green orcs are the orcs that have been reclaimed by barbarism in the aftermath of the Twilight War, usually found living in tribes (alongside their captive - or not - women) in the wilderness, and doing the usual Nine Hordes things. They are the most numerous type of the orcs today.
Black Orcs -The black orcs that the orcs that have been reclaimed by barbarism just as their green orcs brothers, but ended up being picked by the Immutable Hierarchy. The black orcs are strong and disciplined, reproduce quickly and make for great warriors, often forming large parts of the Hierarchy's armies.

Genetics and Reproduction

Orcs are unique when it comes to reproduction - they are an all-male race that is capable of impregnating pretty much every sapient humanoid with a womb. However, the trick is that the children are always orcs, with very limited genetic participation of their mothers which appears to only be responsible for a number of minor traits.
The orcs appear to be responsible for both sides of the reproduction (making them genetic hermaphrodites), with the chosen mothers being merely responsible for gestation and birth of the resulting child, although some less important genes appear to sometimes inherited. Magic is definitely involved.
This is considered a one more proof that they were created as a dedicated warrior species, as such a method of reproduction makes them be able to damage their enemy's demography while simultaneously spreading themselves. Which is how the Sekarkan Empire planned to limit the numerical superiority of the non-ancient dragon members of the First Empire.
This has made them understandably unpopular during the pre-imperial and early imperial times. This has mostly changed in the modern times due to the Second Era Empire gathering all the surviving orcs (most of which has to a various degree switched to consensual reproduction due to lack of wars in that era) and gave them a very detailed (and long) lecture concerning the issues of consent to sex and how willing and well-treated mothers equal healthy and strong children (even when those are orcs). Although it has to be said that some hereditary mindsculpting (or mind magic in general) might have been involved in order to truly convince the orcs as a species.
The result of that is the modern orcs mostly getting the message, sometimes to a rather hilarious degree - it's not unheard of for the poorer women of the borderlands to actually abandon the civilization (and their past lives) and go to live with nearby orcish tribe, simply due to the way they will be treated. A lot of food (especially meat) alone is a good motivation, especially in the times of the famine.
Of course, orcs are still mortals - despite the fact that their culture has such values, there are cases of orcs (sometimes group of them) that act against it. Then again, it doesn't make them any different from normal humans.
As a sidenote (that confirms the idea behind the creation of the orcs), the orcs normally aren't capable of impregnating anybody, regardless of their sexually activity. Actually forcing themselves into fertility (which is pretty much a self-imposed heat) typically requires a lot of adrenaline (either by hunting something large or by fighting in the war, typically).
Once achieved, this state lasts for at least a week or two, with every sex in that period ending with pretty much guaranteed pregnancy unless the woman is infertile or during a period.

Dietary Needs and Habits

Orcs are carnivores. They eat meat. Mostly - they can partially replace it with other things (preferably fish, then mushrooms and finally normal plants) but sooner or later they will start feeling weaker and weaker. They also enjoy bear and strong alcohols, but are rather displeased by wine, especially if its diluted in water.
One of the greatest disagreements between the Grand Empire of Karadia and orcs is that they are rather fervent cannibals. Consumption of the enemies they defeated in battle is quite common (especially in large tribes that still adhere to their traditional culture, those that assimilated unto other cultures do not commit acts of cannibalism (though they still might feel urge to do that after particularly fervent battle).
Due to the way their culture works, they specifically target the enemies that proved their strength, especially if they were the ones that that defeated them.

Additional Information

Perception and Sensory Capabilities

Created by the goblins to fight against the dragons and their worshippers on the surface, they were made with surface-living in mind. As a result, they have incredible eyesight (the Second Era Empire' had them serve as snipers primarily, at least before the Warforged were created), very good hearing and a sense of smell way above the human average. In the end, trying to sneak up on them is extremely hard.