"Innocence proves nothing."
The internal security apparatus of the Imperium of Man, the Inquisition was founded at the end of the Horus Heresy and tasked with rooting out traitors and preventing rebellions. Inquisitors have extremely broad authority to carry out their work and are one of the few groups granted permission to call Exterminatus, the condemnation and subsequent destruction of entire worlds, and have gained a reputation for being quick to write planets off. As such, they are some of the most feared individuals within the Imperium.
The Inquisition is divided into three primary camps: The Ordo Hereticus, which investigates and roots out heretical and general rebellious activity within the Imperium, the Ordo Xenos, which investigates the numerous alien races that oppose the Imperium and work to counter their influence, and the Ordo Malleus, which investigates affairs concerning psykers, the Warp, and Chaos. Each has a "Chamber Militant" consisting of specialized armed forces. These, respectively, are the Adepta Sororitas, the Deathwatch, and the Grey Knights. Numerous minor ordos exist, and the Inquisition is divided between equally numerous different schools of thought concerning how they should go about their duties.
Their icon is a stylized "I" with a skull in the middle, often written in text as "=][=".