Humans are the most populous race in the Warhammer 40,000 setting. They originated from the planet Terra and spread from the planet aeons ago. Most are anatomically identical to real life humans, the others being either cybernetically or genetically enhanced or warped by Chaos.
Some time long ago humanity left Terra. They colonised their solar system and beyond, creating the first Human Empire.
During this time the Eldar fall occurred and Slaaneshs birth cry ripped the souls of most of the galaxies occupants including those of humans. They galaxy must plunged into darkness and daemonic warp storms meant that inter-planetary travel was made impossible. Human worlds became isolated, humanity would never be truly united again. Some human mutated into subspecies, others advanced while others regressed, some intermingled with other species while others worshiped new darker gods.
Terra was plunged into an era of civil war known as the Age of Strife, at time were cliques run by warlords fought for control. One man, the one who would later become the Emperor of Mankind, started a military campaign known years later as the Unification Wars. He fought the many factions of Terra; bring the planet to heel with his super soldiers (precursors to the Space Marines). We proclaimed himself Emperor of Mankind and announced the Great Crusade. One by one, world after world the Imperium of Man grew. It was in this time mankind encountered xenos species which they vehemently purged. Only matched by the Eldar and Orks.
As the Imperium of Man grew the Emperor chose to resign back to Terra leaving the generals of his new Space Marine legions, the Primarchs in charge. He chose Horus of the Luna Wolves to be become general supreme, the Warmaster.
Horus proved to be weak and was corrupted by Chaos. Diverse circumstances meant that many of his brother Primarchs also rebelled. This started the 2,000 year long civil war known as the Horus Heresy. Humanity was again plunged into an era of genocide, carnage and slaughter. The war ended in Horus's death and a near fatal injury of the Emperor.
Like real humans but most have minor alterations.
The vast majority of humans live under the Imperium of Man but few live outside its domains. Planets both known and unknown to the Imperium are ruled by Humans independent of the Imperium. A side from these planetary empires such as the Tau and the Lost and Damned exist.
Some humans are ruled by the Tau, they are called Gue'vesa in the Tau language. They descend from those who joined to the Tau Empire by either diplomacy or war or, those left behind during the Damocles Campaign.
The Tau term for Gue'vesa soldiers is "Gue'vesa'la," Sergeants is "Gue'vesa'ui," Higher Sergeants are "Gue'vesa'vre", Commanders are "Gue'vesa'el" and High-level Gue'vesa commanders are called "Gue'vesa'o."