Crossbones was the leader of a small-time street gang in New York who made it big after attending a ‘school for criminals’ run by the supervillain Taskmaster. He then worked briefly with the communist Red Scull before switching teams and joining the Nazi Red Scull (the one from the first Captain America movie). Of course, the movie will probably simplify his back-story somewhat.
As the title suggests, the primary villain of the film will be the Winter Soldier, played by Sebastian Stan (Bucky Barnes from the first film; who turns evil after a dose of Soviet brainwashing). Samuel L. Jackson, Cobie Smulders, and Scarlett Johansson also make their return to the Marvel Universe as Nick Fury, Maria Hill, and Black Widow respectively.
Anthony Mackie will also be making his Marvel debut as The Falcon, an ally of Captain America who dresses like a falcon and can fly and telepathically communicate with birds.
Captain America: The Winter Soldier comes out April 4th, 2014.