Before the Great War, it was one of the most powerful and heavily armed powers known. Due to rampant corruption and excessive use of force, the Confederacy found itself at odds with many rebel factions. Some of their amoral tactics included experimentation of the Zerg as a weapon using fringe worlds as test beds with Psi-emitters. It was eventually succeeded by the Terran Dominion some time during the Protoss and Zerg invasion.
For more information see the Terran Confederacy on the parent site