In honor of the 6th Edition 40k rules coming out, I present this brief background:

The Imperium of Man

The Imperium of Man is the galactic empire under which the majority of humanity is united. The founder and ruler of the Imperium is the god-like Emperor, the most powerful human psychic to date. Founding the Imperium ten thousand years ago, he continues, at least nominally, to lead it.

The Imperium is the largest and most powerful political entity in the galaxy, consisting of at least a million worlds, which are dispersed across most of the Milky Way galaxy. Consequently, an Imperial planet might be separated from its closest neighbour by hundreds or thousands of light years. As a stellar empire, the size of the Imperium can not be measured in terms of continuous territory, but only in the number of planetary systems in its control.

Several aliens and forces (the forces of Chaos, Tyranids, Eldar, Dark Eldar, Orks, Tau, Necrons) challenge the supremacy of the Imperium. From within, the Imperium is threatened more insidiously by rebellion, mutation, dangerous psykers, and subversive cults.

Without the protection of the Imperium, mankind would fall prey to the countless perils that threaten it.

The Horus Heresy

The Horus Heresy was the first and most devastating civil war in Imperial history. Occurring in M31 and lasting less than a decade, it divided and nearly destroyed the fledgling Imperium. It marked the end of the Great Crusade and the encasing of the Emperor of Mankind into the Golden Throne.2