Imperium
The Imperium of Man is a galaxy-spanning interstellar Human empire, the ultimate authority for the majority of the Human species in the Milky Way Galaxy in the 41st Millennium A.D. It is ruled by the living god who is known as the Emperor of Mankind.
The founder and ruler of the Imperium is the god-like Emperor of Mankind, the most powerful Human psyker ever born. The Emperor founded the Imperium over 10,000 Terran years ago in the late 30th Millennium during the Unification Wars on Old Earth following the terrible period in Human history known as the Age of Strife.
Orks
The Orks, also called greenskins, are a savage, warlike, green-skinned species of humanoids who possess physiological features of both animals and fungi who are spread all across the Milky Way Galaxy.
They are seen by their enemies (pretty much everyone else in the universe) as savage, violent, and crude, but they are the most successful species in the whole galaxy, outnumbering possibly every other intelligent starfaring species, even Humanity (with the very plausible exception of the Tyranids).
Aeldari
The Aeldari Empire, also sometimes anachronistically called the Eldar Empire, was the great interstellar empire of the Aeldari species. The Aeldari Empire was the dominant power of the Milky Way Galaxy in the years after the end of the War in Heaven until the Fall of the Aeldari in the 30th Millennium A.D.
The Aeldari Empire held almost complete control over the galaxy, even during Mankind's Age of Technology, and was characterised by both its own highly advanced technology and the increasingly cruel and hedonistic excesses of its people in its final years.
T'au Empire
The T'au (also spelled Tau in older records) are a young, humanoid and technologically-advanced intelligent species native to the Eastern Fringes of the Milky Way Galaxy.
They are fighting to expand their interstellar empire and extend a philosophical concept they call the "Greater Good" (Tau'va in the T'au Lexicon) to all the intelligent species of the galaxy.
Chaos
Chaos, also known to its servants as the Primordial Truth or the Primordial Annihilator, and to the Imperium of Man as the Archenemy, is the universal and usually malign metaphysical force embodied by the malevolent intelligent entities comprised of psychic energy that live in the Immaterium.
The entities who embody Chaos are mostly Daemons, but the term also encompasses those mortals who have thrown in their lot with Chaos, ranging from simple peasants and/or manufactorum labourers who serve as Chaos Cultists, to Traitor Imperial Guardsmen, planetary militia, Imperial nobles, planetary governors, and even the mighty Chaos Space Marines and Traitor Titan Legions of the Dark Mechanicum.
Tyranids
The Tyranids are an extragalactic composite species of hideous, insectoid xenos. They actually comprise an entire space-faring ecosystem comprised of innumerable different bioforms which are all variations on the same genetic theme.
The Tyranids are unlike any other intelligent species encountered by Humanity. They are the ultimate predators; to them, all living things, from the lowliest insect to the most advanced starfaring civilisation, are mere prey. Only now are the inhabitants of the galaxy realising the scale of the threat; unless the Tyranids can be stopped, it will mean nothing less than the extinction of all life.
Necrons
The Necrons are a mysterious xenos species of humanoid, robotic skeletal warriors that have lain dormant in their stasis-tombs for more than 60 million Terran years, though they have begun to awaken at last. They are the soulless creations and former servants of the ancient C'tan, the terrible Star Gods of Aeldari myth.
The Necrons are ancient beyond reckoning, predating even the birth of the Aeldari. At long last, however, they are beginning to awaken from their Tomb Worlds, for the galaxy is ripe for conquest and the restoration of the Necron Empire since the disappearance of the Old Ones more than 60 million standard years ago.
