The Outcasts were former members of the Brotherhood of Steel. They dedicate their lives to acquiring new technology. They wear black and red armor. Their main base is Fort Independence and you can turn in technological items to them for a reward (only at Fort Independence). You can usually see them wandering around the wasteland in a group of two along with a robot (Robobrain, Protectron or Sentry Bot).They aren't hostile to the player.
For a complete history and more information, visit http://fallout.wikia.com/wiki/Outcast