As a young boy, James Buchanan Barnes lost his father (a soldier during WWII), and was adopted by Camp Lehigh as their mascot, and given the nickname "Bucky". It was here that he learned the identity of Captain America. He underwent rigorous training and was assigned to be Cap's sidekick, accompanying him on many adventures, and the two often worked with the original Invaders. However, on a final mission against Baron Zemo, Bucky and Cap hopped on an experimental drone plane in an attempt to disarm a bomb. The bomb detonated, dropping Cap in the North Atlantic, where he would later be found and thawed out by the Avengers. American forces never found Bucky's body, and he was presumed dead. Unbeknownst to the Americans, he was found and revived by Russian General Vasily Karpov.
When Bucky awoke, he had no memory of his identity, which gave Karpov an opportunity to reprogram Bucky as a Soviet assassin called the Winter Soldier. He was sent all across the globe, committing political assassinations with huge effects on the Cold War. However, his memory implantation caused mental instability, and he was kept in stasis between missions to prevent rebellion.
Recently, the Winter Soldier killed the Red Skull, and stole his Cosmic Cube for Karpov's successor, former General Aleksander Lukin. In order to mess with Cap's mind, Lukin also commanded the Winter Soldier to kill Jack Monroe and launch a major terrorist attack on the city of Philadelphia, fueling the Cosmic Cube's power in the process. However, when the Winter Soldier kidnapped S.H.I.E.L.D. agent Sharon Carter, she told Captain America upon her rescue that she believed her captor to be Bucky. Later, Nick Fury confirmed to Cap that the Winter Soldier was indeed his former partner.
When Lukin ordered the Winter Soldier to bury the Cosmic Cube in an underground bunker, Cap, with help from the Falcon and Iron Man, tracked him down and defeated him. After the battle, Cap used the Cosmic Cube to restore Bucky's memories, but he then destroyed the Cube and disappeared. All but Captain America believed him to be dead.
It was recently revealed that the Winter Soldier was involved in helping Wolverine escape from Weapon X, and was responsible for the loss of his wife and baby.
When the Superhuman Registration Act was passed, S.H.I.E.L.D's Nick Fury recruited Bucky for covert operations, so as to protect Captain America from government forces. His current whereabouts are as yet unknown.