Inside of the Imperium of Mankind on Holy Terra, the God-Emperor of Mankind rests upon the Golden Throne, and has been for 10,000 years. It is unknown to the common man what occurs on Terra and what the God-Emperor of Mankind truly does, but any devout devotee to the Imperial Creed knows that the God-Emperor is simply managing all of His vast planets and armies under His mighty hand from the Golden Throne of Terra.
Humans inside of the Imperium can vary widely, from feral worlders that have only seen Imperials when they retrieve their military tithes from villages, to the nobles of hive worlds, sprawling cities condensed atop of itself to form mighty spires that can reach the clouds.
The majority of humans within the galaxy reside within the Imperium, or within its borders. Due to this, even if you are not born in Imperial space, most humans know of the Imperium: what it stands for, the Imperial Creed that reveres the God-Emperor, and its vast hands that stretch over millions of planets and untold trillions of people.
Chaos. Since the Horus Heresy and the Long War, humans have been lured to the powers of Chaos, whether it be by subjugation of their planets, or because they rather be free from the iron hand of the Imperium. Humans that fight under the forces of Chaos are oftentimes under the leadership of a warband, a dark apostle, or an apostate. Although the forces of Chaos are able to form competent societies and some forms of civilization, this is rarely the case, as their innate calling to the Ruinous Powers cause them to seek greater journeys, for glory or power.
Tau. Under the Tau Empire, humans are able to live soundly in a caste known as gue'vesa. Under this system, they are treated similarly to second-class citizens, but are not necessarily treated unfairly. Many humans are sent off in propaganda fleets to attempt to garner other humans towards their cause.