The Imperium of Man is a galaxy-spanning interstellar human empire, the ultimate authority for the majority of the human race in the Milky Way Galaxy in the late 41st Millennium A.D. It is ruled by the living god who is known as the Emperor of Mankind. However, there are other humanoid species classified as Imperial citizens, mainly mutant offshoots of genetic base-line humans who are known as Abhumans and include such human sub-races as the Ogryns, Ratlings and Squats.
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. The Emperor continues, at least nominally, to rule the Imperium as both its political master and its primary religious figure.
The Imperium of Man is a war-torn stellar empire, teetering on the brink of collapse. For 10,000 Terran years it has been ruled by the deathless Emperor, a being of almost limitless psychic power, to whom thousands of souls are sacrificed daily. The peoples of the Imperium live in a place where daemons are real, mutation is frequent and death is a constant companion. To be alive in the late 41st Millennium is to know that the universe is a terrifying and hostile place. It is a place where you are but one amongst billions and, no matter how heroic your death, you will not be missed. A truly vast domain, the Imperium is spread amongst the many stars of the galaxy. Its territories encompass untold millions of stars and countless more human lives. In its name, terrible wars are fought and desperate sacrifices made, yet even this river of carnage and blood is a small price to pay, for the Imperium is the guardian of Mankind. Were it to pass into nothingness, so too would the human race, destroyed by enemies uncountable, to the braying laughter of the Dark Gods.
The Imperium is the largest and currently most powerful political entity in the galaxy, consisting of at least 1,000,000 human-settled worlds dispersed across most of the Milky Way Galaxy. Consequently, an Imperial planet might be separated from its closest neighbour by hundreds or even thousands of light years. As a stellar empire, the size of the Imperium cannot be measured in terms of contiguous territory, but only in the number of planetary systems under its control. However, most humans in the galaxy have little day-to-day contact with the government of the Imperium unless they serve in one of its Adepta or run afoul of its various protectors, such as the Inquisition or the Adeptus Arbites.