Orcs are related to goblins. The brutish orcs, though less intelligent, tend to dominate the meeker goblins. Orcs are not very intelligent or knowledgeable but are capable of cunning. In combat they can transform even the most common object into a lethal killing instrument. They are extremely warlike, with a society geared towards constant warfare, living in small tribal groups with a strong chain of command. They are known for large, brutal armies of difficult-to-kill troops, usually supported by a variety of units and war machines. The constant need to fight keeps the orcs from forming anything but temporary alliances with one other.
Physiology
Orcs are taller and broader than humans, with short legs and long arms much like an ape. They have massive heads which come directly forward on their necks, giving them a stooping appearance. Their jaws are lined with vicious fangs that jut out from their underbite. They have beady red eyes, a generally foul demeanor, and are naturally bald. Normally six feet tall, they are up to seven feet tall when stretched out of their characteristic stoop.
They have tough thick green skin which is highly resistant to pain. They can survive even the most horrific of injuries. Orcs have an "adrenaline-fueled metabolism", which means that they grow in size, strength, endurance and belligerence as they fight and defeat opponents. An orc will only stop growing when he finds an opponent that he can't beat.
Society
Orc "culture" is dominated by the concept of strength. There is no law, and therefore no crime. If an orc feels wronged, he can challenge the source of his grievance to a pit fight. Once the fight is over, the matter is considered settled. Orcs have no concept of pity; to an orc, the weak are simply weak, and are beneath them in society.
An orc attains a higher social rank by fighting and defeating members of that rank. Because of their bizarre physiology, orcs receive a steady supply of adrenaline when preparing to challenge a superior, causing him to physically increase in size and muscle-mass. An alert superior will spot this occurring and promptly beat the challenger down before he becomes a threat. If the superior fails to notice this development, then a battle inevitably results. Once such a leadership battle is won, the winning orc undergoes an extreme boost in power, gaining up to a dozen pounds of muscle in the wake of the fight. Orc veterans are known as "big'uns", because they are literally bigger and stronger than ordinary orcs. Grimgor Ironhide, a Orc Warlord, is over 15 feet in height. As orcs grow older and beat more powerful foes, they can become "bosses," "big bosses," and eventually "warbosses". A warboss leading several tribes in a Waaagh! is known as a "warlord". Killing a warlord does not stop a Waaagh, it merely causes the warlord's lieutenants to fight each other to decide who the new warlord will be.
Orcs communicate through their own language which has been described as a variety of guttural sounds, but they are often shown as capable of speaking humans languages, mostly broken and fragmented. There is one tribe of savage orcs that is said to have a vocabulary consisting entirely of the word "Waaagh!," with variants in volume, pitch, and accents making up for individual words. Their written language works in a similar way to Egyptian hieroglyphics, only louder and cruder.
Orcs tend to ally with Goblins, but their alliance is more of a matter of the orcs bullying their smaller Goblinoid cousins into being everything from servants, to human (goblin) shields, to an emergency food source. Their warbands also often include giants, trolls, ogres and other monsters in the same vein.
The Waaagh!
Waaaghs are a cross between a mass migration, holy war, looting party and pub crawl, with a bit of genocide thrown in for good measure. Millions of Orks will gather together, drawn to the power of a single dominant Ork called a Warboss, who is larger and more intelligent than the Orks around him. Then the Orks will set off to find an enemy to fight - any enemy. Ork Waaaghs will sweep whole planetary systems away and destroy armies and fleets in tides of bloodlust and carnage, and only once the Orks have killed every available enemy will they start to fight amongst themselves again. In some cases, Warlords eventually forge their Waaagh! into an empire, although very different then a normal empire.
The Waaagh! is also used to refer to an Ork Warboss' army.