Orks are a xenos race found throughout the galaxy. They are a unique species that is part fungus, allowing them to reproduce simply by shedding spores. They are also intrinsically psychic, though instead of using the Warp, they use their own collective willpower so that whenever a large number of them gather in one place, they become more powerful. This is known as a WAAAGH! And grants the Orks strange reality-bending powers, such as making anything painted red automatically go faster, or allow Orks to grow in size and strength for every victory in battle. They also have technical know-how programmed into them: this, in combination with their gestalt psychic abilities allows them to turn otherwise useless junk into deadly weapons of war.
The Orkoid genus includes various other creatures that are grown from the same fungal spores, such as the diminutive gretchin and snotlings used for menial labor and cannon fodder. Squigs are orkoid creatures used for food, hunting animals, or even massive war beasts.
The Orks were created by the Old Ones to fight on their behalf during the War in Heaven against the Necrons. The Orks (then known as Krorks) possessed a unique gestalt psychic field, drawing power from each other rather than the warp itself. They were also engineered to have an innate knowledge of technology and to grow from spores, making them into an ideal self-sufficient fighting force. However, when the Old Ones were eventually destroyed by Chaos, the Orks, not having been created with the ability to stop fighting, ran amok and continued to fight everyone in the galaxy indiscriminately.
The great Ork empire of Ullanor was defeated by the Emperor's forces during the Great Crusade, but they came back with a vengeance under the leadership of the Beast, the biggest and most powerful Warboss in history. Massive Ork forces have also attacked Armageddon twice, and are currently engaged in an endless fight against the Tyranids at Octarius. The Orks live to fight, and seek only to conquer and loot any world they encounter.
Like the Eldar, the Orks have their own gods: Gork & Mork. They represent everything orky and seek to make them the dominant species in the galaxy.
Orks are almost identical to how they appear in canon as they are already inherently ridiculous. However, they are also mentioned in violently depraved smut (such as Typhus' reading of Squad Broken, or Kryptman's "research file") despite Orks not possessing genitalia.
After Vulkan's sacrificial death trying to defeat the Beast, wherein he tackled his enemy into an Ork reactor which run on Ork psychic energy, upon his return it was shown that his consciousness had become partially fused with that of the Orks. This causes Vulkan to experience sudden and instant personality shifts manifested by changes in his voice and mannerisms.
Because Orks are part fungus, eating their flesh causes a psychedelic reaction, especially to Space Marines due to one of their organs allowing them to see the memories of their fallen foes. This affected the Flesh Eaters Space Marine chapter even more so; after consuming an Ork which caused Sanguinius to appear before them demanding "heretic burgers" the chapter began their eternal quest to eat everything in sight.