The Orks are a warlike composite race of humanoids collectively known as "Orkoids". The Orkoid species share a unique biochemistry incorporating a second DNA strand resembling that of Terran algae or fungi, and this trait manifests as an exceptionally tough and adaptable physiology featuring vivid, green-pigmented skin. The dominant species, Orks, are considerably larger and more muscular than either Humans or Eldar, and are in comparison highly aggressive and brutish. They are often characterized as stupid, but they do in fact possess roughly the intelligence of an average human. It is their more direct methods of problem solving, and lack of curiosity or philosophical tendencies that can make them appear unintelligent.
The Gretchin (known as "Grots" to the Orks) are far smaller and physically weaker, and much less aggressive, and as such are treated as a slave race by the Orks. They do however display considerable intelligence and are highly adept tinkerers, looters and thieves. Often thought of as cowardly (and often are), they can nevertheless show a highly vicious temperament when confronted, and like the Orks are not to be trifled with, particularly in numbers.
In the novel Kill Team (Thorpe, 2001), some Orks are seen operating as mercenaries for hire. This is usually a temporary situation, as Ork mercenaries will be just as happy to fight their current employers at a later date. They are stronger in melee combat than at ranged. They also are very weak when it comes to getting hit but the melee damage and the sheer amount of their forces makes up for that.
Read more about this topic: Warhammer 40,000 Species