Let's start with the obvious. Originally the childhood friend and sidekick of Steve Rogers, Barnes was presumed dead, along with Steve, at the end of World War II. However, he was actually captured by the Soviets and turned into the deadly assassin known as the Winter Soldier. He was
eventually saved by Steve and was able to rebuild his life.
After the apparent death of Steve at the hands of an assassin, Bucky was chosen to take up the mantel of Captain America. During his time as the good captain, he helped to repel the Skrull’s Secret Invasion and prevented the spread of a virus that could have wiped out half the human race. Eventually, it was discovered that Steve hadn’t actually died, and after his rescue, Steve and Bucky both helped defend Asgard in the Siege storyline. In the end, Bucky was stripped of the title of Captain America due to fears that his past could taint the image.
Civil War will mark the third outing for Sebastian Stan in the role of Bucky Barnes. Though it's unclear (read: unlikely) whether or not any main character will bite the big one in this one, if Chris Evans' Steve Rogers is indeed killed, the smart money's on Barnes to take his place.