Cerberus was a human-survivalist paramilitary group led by the enigmatic Illusive Man. Cerberus's core belief was that humans deserve a greater role in the galactic community, and that the Systems Alliance is too hamstrung by law and public opinion to stand up effectively to the Citadel races. Any methods of advancing humanity's ascension were justified, including illegal or dangerous experimentation, terrorist activities, sabotage and assassination. Cerberus operatives accepted that these methods are brutal, but believed history would vindicate them. Nevertheless, both the Systems Alliance and the Citadel Council declared Cerberus to be a terrorist organization and will prosecute identified Cerberus agents accordingly.
By the time of the Reaper War, Cerberus had grown in power to the point that it was almost a rogue state. See: Cerberus in the Reaper War.
Following the Reaper War, Cerberus is defunct, although neo-Cerberus groups certainly exist (for example, Anagenesis). These lack the incredible resources that the Illusive Man was able to command.