Warhammer 40K Races
Chaos
From the Warhammer 40k Lexicanum, "Ten thousand years ago, the Horus Heresy nearly tore the Imperium apart. The daemonic Gods of Chaos corrupted half the Space Marine legions, and plunged the Imperium into a cataclysmic civil war. The Imperium defeated the traitors, but at great cost.
The Forces of Chaos still tear away at the Imperium. The sinister whispers of the dark gods turn many people away from the Emperor and their own people, and the remnants of the traitor Space Marine Legions still reside in the Eye of Terror, occasionally striking out in what is known as the Black Crusades."
Eldar
From the Warhammer 40k Wikia, "Trust not in their appearance for the Eldar are as utterly alien to good, honest men as the vile Tyranids and savage Orks. They are capricious and fickle, attacking without cause or warning. There is no understanding them for there is nothing to understand -- they are a random force in the universe." — Imperial Commander Abriel Hum
Necrons
From the Warhammer 40k Wikia, "The Necrons are a mysterious race of robotic skeletal warriors that have lain dormant in their stasis-tombs for more than 60 million Terran years and who are the soulless creations and former servants of the ancient C'tan, the terrible Star Gods of Eldar myth. The Necrons are ancient beyond reckoning, predating even the birth of the Eldar. 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. The Necrons are a completely robotic humanoid species whose technological prowess is probably unmatched by any of the other intelligent species of the galaxy."
Orks
From the Warhammer 40k Wikia, "The Orks, also called Greenskins, are a savage, warlike, green-skinned race of humanoids who are spread all across the Milky Way Galaxy. They share many features with Warhammer Fantasy Orcs (and were initially called "Space Orcs" to distinguish them). They are seen by their enemies (pretty much everyone else in the universe) as savage, warlike, and crude, but they are the most successful species in the whole galaxy, outnumbering possibly every other intelligent race, even Mankind (with the very plausible exception of the Tyranids)."
Astra Militarum
From the Warhammer 40k Wikia, "The Imperial Guard, also known as the Astra Militarum in traditional High Gothic, is the largest coherent fighting force in the galaxy. They serve as the Imperium of Man's primary combat force and first line of defence from the myriad threats which threaten the existence of the human race in the late 41st Millennium. It is comprised of countless billions of men and women -- hundreds of thousands of different regiments, supported by a vast array of light and heavy armoured vehicles that provide the Guard's primary offensive punch. They are usually the first Imperial force to respond to a threat if a planet's Planetary Defence Force (PDF) fails to suppress it."
Sisters of Battle
From the Warhammer 40k Wikia, "The Adepta Sororitas, also known as the Sisters of Battle and formerly known as the Daughters of the Emperor, are an all-female division of the Imperial Cult's ecclesiastical Adepta known as the Ecclesiarchy or, more formally, as the Adeptus Ministorum. The Sisterhood's Orders Militant serve as the Ecclesiarchy's fighting arm, mercilessly rooting out corruption and heresy within humanity and every organization of the Adeptus Terra. There is naturally some overlap between the duties of the Sisterhood and the Imperial Inquisition; for this reason, although the Inquisition and the Sisterhood remain entirely separate organisations, the Orders Militant of the Sisterhood also act as the Chamber Militant of the Inquisition's Ordo Hereticus. "
Imperial Knights
From the Warhammer 40k Wikia, "An Imperial Knight is a small combat walker that is a one-man version of a Titan. However, a Knight is smaller and less capable in combat than even the smallest class of true Titan utilised by the Titan Legions, the Warhound-class Scout Titan. Knights normally deploy in squadrons comprised of several of these war machines. These ancient battlesuits stand thirty to forty feet tall, are protected by nearly impenetrable Ion Shields and armed with a devastating array of heavy weapons. Imperial Knights are colossal war machines that tower over the battlefield. They are propelled into battle by powerful servos and have the power to kick over tanks and crush soldiers into red paste."
Adeptus Astartes
From the Warhammer 40k Wikia, "The Space Marines or Adeptus Astartes are foremost amongst the defenders of humanity, the greatest of the Emperor of Mankind's warriors. They are barely human at all, but superhuman; having been made superior in all respects to a normal man by a harsh regime of genetic modification, psycho-conditioning and rigorous training. Space Marines are untouched by plague or any natural disease and can suffer wounds that would kill a lesser being several times over, and live to fight again. Clad in ancient Power Armour and wielding the most potent weapons known to Man, the Space Marines are terrifying foes and their devotion to the Emperor and the Imperium of Man is unyielding. They are the God-Emperor's Angels of Death, and they know no fear."
Tau
From the Warhammer 40k Wikia, "The Tau are a young, humanoid and technologically-advanced intelligent race native to the Eastern Fringes of the Milky Way Galaxy who are fighting to expand their interstellar empire and a philosophical concept they call the Greater Good. The Tau claim to be a peaceful race when possible, asking if others will join their cause voluntarily instead of fighting. However, if their peaceful overtures are refused, the Tau may well decide to conquer a planet and add it to their growing interstellar empire for the greater good, searing the flesh from the bones of anyone who stands against their benign expansions. Tau society is divided into a number of castes, each responsible for managing a specific aspect of their society. The Tau's central motivating ideal is that everyone in their empire regardless of their species will work for the collective betterment of everyone else, an almost mystical philosophy they call the Greater Good."
Tyranids
From the Warhammer 40k Wikia, "The Tyranids are an extragalactic composite species of hideous xenos that is actually a space-faring ecosystem comprised of innumerable different bioforms which are all variations on the same genetic theme. The Tyranid race is ultimately dedicated solely to its own survival, propagation and evolutionary advancement. The Tyranids collectively form a monstrous superorganism that travels across the universe in their great Hive Fleets of biomechanical Hive Ships, systematically consuming all other biomatter to enable its own rapid evolution and reproduction. All Tyranid organisms are synaptic (psychically-reactive), and each Tyranid creature within a Hive Fleet shares and contributes to a communal Hive Mind, which allows the trillions of beings comprising the Tyranid Hive Fleets to communicate and organise instantaneously on a staggering scale."