The Orks, also called Greenskins, are a savage, warlike, green-skinned species of bestial, asexual humanoids who are spread all across the galaxy. They are unique in that they possess the physiological features of both animals and fungi.
Orks are seen by their enemies (pretty much everyone else in the universe) as primitive, barbaric, hyper-violent, and crude, but they are the most successful and widepsread intelligent species in the whole galaxy, outnumbering possibly every other intelligent starfaring species, even Humanity (with the very plausible exception of the Tyranids).
The Orks' unquenchable thirst for battle has always proved their downfall: historically, the Ork tribes have spent much of their time fighting amongst themselves, waging brutal wars with only the strongest surviving. On occasion, an Ork leader will emerge who is mighty enough to defeat his rivals and unite the warring tribes. His success draws other tribes to him, and soon a great WAAAGH! is underway -- partly a migration, partly a holy war that can exterminate the populations of entire star systems.
When the Orks are on the rampage, the galaxy trembles, and in the Age of the Dark Imperium there are more WAAAGH!s rising than ever before recorded.