Humans are the most populous of all the races in the Warhammer universe. It doesn't matter if it is a 40,000 game or one set in medieval times, humans play a large part of whatever age they are a part of, tending to play the part of the race of "order".
 Warhammer Games
No matter whether it is against Chaos, or the forces of Destruction, humanity is always at the center of the conflict, their empires representing Order amongst worlds slowly being torn apart by torrents of magic, constant war and the undercurrents of chaos. When in these ages, typically you will see humans ride out with races such as Dwarves or High Elves, races which also tend to represent order and strive to reign in growing Chaos, a concept which is common amongst the non-40,000 Warhammer games.
 Warhammer 40,000 Series
In the 40,000 series, which is set tens of thousands of years in the future, humanity plays a central role as the Imperium of Man, the central empire in the Milky Way galaxy which holds sway over a plethora of worlds, millions in total. Although they share it with other races like the Orks, humans are by far the most numerous, and perhaps this massive population has played a part in putting them on edge with neighbouring civilizations. Most of the soldiers fight on the front of the Imperial Guard.
Humans of this age are involved in genetic modification, giving them greater abilities over normal humans. The finest examples of these genetically modified humans are the Sisters of Battle and the Space Marines.