The Aquila - the symbol of the Imperium of Man
The lore and back story behind Warhammer 40k is vast to say the least. Official canon is derived from the various rulebooks and 'army'-specific codexes as well as the vast host of novels written on the subject. Known as the Black Library, somewhere around 100 books have been published by a handful of authors to fill the racks of lore and knowledge about this expansive and detailed world. To tell the story of the 40k setting would take many pages, but I shall try and compress it into a 'semi' short read. Perhaps I'll make more detailed posts later...
Holy Terra (Earth) - the center and origin of humanity
The old days...
As the name of the franchise implies... we are presently around the year 40,000. Faster than light travel, laser guns, aliens, and all that good stuff exists in some form. So how'd we get here?
By the year 15,000 mankind had reached the peak of its technological innovation. Man had spread throughout the stars of the Milky Way galaxy. All was well for another 10k years until the Age of Strife, around the year 25,000. The method for human (faster than light) travel and communication relies on an entity known as the Warp. It is an alternate reality that persists everywhere - ships (aided by 'Navigators') and telepathic messages (sent by 'Astropaths') enter the Warp temporarily for fast travel. However, the Age of Strife is marked by massive Warp-storms throughout the galaxy. These storms disrupt travel and communication, isolating most of humanity into individual planets or small clusters of planets. With a cessation of trade and communication comes conflict. Battles over "agri-worlds" (which focus on food production for the highly populated city-worlds who can not sustain themselves) and other resources ensues. Individual planets see strife over local control. TL;DR - shit is bad. Humanity is scattered.
Brave soldiers of the Emperor - the Imperial Guard
By the year 30,000 the Warp Storms subside and a single force unifies Terra. The new master of Earth is known only as the Emperor. Using genetically modification he enhanced his soldiers into 'Space Marines' - demi-god like in power, these warriors are physiologically superior to any normal warrior. With the Space Marines at the forefront of his army, followed by untold billions of normal soldiers of the Imperial army the Emperor set off on his Great Crusade to unite the stars.
Space Marines of the Dark Angels Legion
Heading the Space Marine legions are the Primarchs. The Primarchs are Space Marines themselves; but are the direct 'sons' of the Emperor - vastly superior to their normal marine brothers. These 20 primarchs each head their own legion of marines. 200 years after the launch of the Great Crusade humanity is almost unified. However, Horus - the most favored son of the Emperor and head of the primarchs was corrupted by Chaos: an evil entity that thrives in the Warp. Corrupted, Horus talks many of his brother Primarchs and their space marine legions into joining his cause to rebel against the Emperor. After bitter warfare, Horus and his allies failed, and retreat into the Warp. The Emperor is left mortally crippled - interned in the Golden Throne: a life sustaining device. While the Emperor is not dead, he also does not live. Only his mind remains alive.
The God-Emperor of Man before his internment on the Golden Throne
The year is 40,000. The Emperor still remains on the Golden Throne. The High Lords of Terra - a council of the Imperium of Man's most senior leaders, rules in his stead. Humanity is beset on all sides by threats - internal and external. From within, chaos corrupts the minds of men - heretical chaos cults and rogue planetary governors rebel. From outside, various alien species threaten the Imperium - Orks, Eldar, Tau, Tyranid, and Necron. All with their own back-story and lore.
The Emperor now; interned on the Throne in a not dead, yet not fully alive state.
That's all for now... I'll post more lore later!!