Around the 25th Millennium, humanity has spread itself across the stars and colonized the galaxy, utilizing beacons to enter an alternate dimension known as the Warp to perform faster-than-light travel. When warp storms caused the beacons to become useless, humans dispersed across the galaxy became stranded and civilization fell to its knees. For the following five millennia, mankind was plagued by Daemonic-incursions, xeno slavers, barbarian warlords and abandoned to their own fate up until the emergence of the Emperor of Mankind, an immortal born in 8,000 BC in Central Anatolia that possessed vast psychic abilities that silently observed from the shadows directing mankind up until he decided to take a more direct role, by forcibly uniting all warring nations on Earth under his banner.
He proceeded to create the Astronomican beacon, powered by his own psyker abilities that would allow FTL travel once again and led the Great Crusade to reunify mankind under his enlightened new order. But just when a new golden age seemed imminent, the newly-forged Imperium was wracked by civil war as half of the Emperor's sons turned against him. The Horus Heresy was ultimately quashed, but at a terrible price: countless worlds were left in cinders, untold trillions were dead, and the Emperor himself was mortally wounded and forced to "ascend" to the Golden Throne, an arcane life-support machine.
The Imperium of Man is a highly militaristic, xenophobic, religiously-fundamentalistic, totalitarian dystopia that combines aspects from the Roman Empire, Nazi Germany, Imperial Japan, Catholic Church and 1984's Oceania. Its the largest, most powerful political entity in the known galaxy, consisting of at least 1,000,000 planets spread across the Milky Way and with at least trillions of inhabitants each, making their total number literally countless. Its form of government is a theocratic oligarchy with neo-feudal tribute system, allowing its member worlds to largely govern themselves as long as they recognize the authority of the Emperor and His servants and support the state religion, the Imperial Cult, which holds the Emperor to be the one, true God of Mankind.
Its a war-torn empire on the brink of collapse, under constant attack of xenos and Chaos, its greatest rival powers in the galaxy who are increasingly challenging the supremacy of the Imperium in the galaxy. In the present, science and human progress have essentially been halted in service to the need to simply maintain the crumbling status quo. From within its own creaking and increasingly stagnant and repressive edifice, the Imperium is threatened more insidiously by rebellion, mutation, dangerous psykers and subversive Chaos Cults. However, despite its myriad problems, without the admittedly authoritarian and often harsh protection of the Imperium, Mankind as a whole would have fallen prey to the countless perils that threaten it. Without the Imperium of Man and Mankind's faith in the Emperor who guides it, the human race would have become extinct long ago.