A far-flung empire of Humanity in the far future, the Imperium is currently in it’s third incarnation lasting about 1113 years or so. It contains an aggressive military consisting of a well trained Navy called The High Guard, an occupation force called The Imperial Guard, and an exploration arm called The Scout’s. Also the Imperium has their own investigative force called The Inquisition and, as a rule, is very distrustful of any alien race that is different from them that they can’t “control”.
That said, there are other alien races surrounding the Imperium. Some the Imperium “get along with” (most everyone), some are “servants” (the Varger), and others are antagonistic with the Imperium. (the Zhodani) Recently a new threat has entered the galaxy causing several of the races to become endangered and making all of them “temporary allies” against the common foe.