The Secret Avengers are a black-ops branch of the Avengers. They were assembled by Captain America (Steve Rogers) to clean up the messes left by Norman Osborne's 'Dark Reign'. The team became a covert branch of S.H.I.E.L.D.
The team share a name with Captain America's Resistance in the Civil War, nicknamed the Secret Avengers. The underground team of heroes was founded by Captain America in response to the Superhuman Registration Act, for those who wanted to fight against Iron Man's initiative for superhuman registration.