Orks, also called Greenskins, are a warlike, green-skinned humanoid race spread all across the Milky Way galaxy. Orks are savage and crude, yet the most successful species in the whole galaxy, possibly outnumbering every other sapient race. However, this massive population is split into hundreds of tiny empires often warring between themselves. It has been speculated that were the Orks ever to unite as a single racial entity, they would utterly crush all opposition. Luckily, the Orks enjoy killing each other just as much as other species.
In Warhammer 40,000, Orks are generally considered a "comic relief" faction with humorous behavior, language and appearance as well as ramshackle technology with amusing names. This, however, doesn't stop them from being a massive threat for the Imperium of Mankind.
Biologically, the Orks are a strange animal-fungal hybrid that reproduces by airborne spores. This makes them extremely hard to eradicate, and grants them exceptional toughness and regenerative ability. Orks can actually heal perfectly well after dismemberment – even decapitation – if the missing part (or something functionally equivalent) is only put back in soon enough. This unique physiology makes them extremely effective warriors.
Orks thrive on conflict and produce growth hormones during conflict, meaning that they grow physically larger and more powerful the more battles they survive. The biggest Ork will lead the group; if another Ork gets even bigger, he will usually kill the old leader and take over. The largest Orks, the "ruling caste", are called Nobz ("nobles"), but the leader of a group is usually referred to as a Warboss.
All Orks generate a species-unique psychic field known as a "Waaagh!" which allows them to instinctively recognise the most well-developed and thus powerful Orks and classify each other in a relative hierarchy of size and strength, which allows them to organise into their characteristic hordes (also known as Waaagh!s) while limiting the infighting necessary to establish their dominance-based hierarchies. Only in situations where the hierarchies become ambiguated, such as in the immediate aftermath of the death of a Nob or Warboss, or otherwise in situations where the Waaagh! field itself does not directly correspond to who is able to take and hold power through violence, do Orks tend to do battle amongst themselves. Thus, Orks can frequently be observed to fight among themselves in small groups, over small stakes of power and control, even while these small groups are co-operating in larger efforts; major warfare and revolutions only take place when Orks, usually larger, more experienced Nobs, percieve through their own cunning that serious gains might be made through open violence.