The Kingsguard, a.k.a. the White Cloaks, are sworn to protect the king and obey him in everything. Under Joffrey's reign, this can often mean beating up helpless girls or killing unarmed peasants, as much as fighting enemy forces. When knights join the order, they relinquish the right to own lands or father children, much like the Night's Watch.
During A Clash of Kings, their ranks include these guys: