While the comic book version of the story had Bucky Barnes, Captain America’s sidekick during World War II, found and revived by the Russians, it appears that in the new film the Winter Solider is serving new masters.
According to the interviews in the featurette below, Bucky is working for Hydra in the new film, the Nazi-like terrorist organization often at odds with the heroes of the Marvel universe.
This fits in perfectly with two other bits of information. We know that Thomas Kretschmann has signed on to play Hydra founder Baron Von Strucker in Avengers: Age of Ultron. Rumors persist that the Scarlet Witch and Quicksilver, two prominent Avengers set to be introduced in the 2nd Avengers movie, will be working for Hydra when that movie begins. Since Hydra is already showing up in Captain America: The Winter Soldier, that also lends credence to the rumor that Scarlet Witch and Quicksilver will actually debut in a post-credits sequence in the new Captain America film.
Here is the featurette. You can also catch a very brief glimpse of S.H.I.E.L.D. agent and oft-times love interest of Captain America, Sharon Carter, in the clips.