In the comics it is well-known that the Winter Soldier is Bucky brought back from "death" by the Soviets and brainwashed to be their assassin. While the Winter Soldier in next year's Captain America sequel seems to be the same character from the comics Marvel Studios is infamous for making changes to either characterization, or origin. These are my theories about who exactly the Winter Soldier is.
Brainwashed HYDRA soldier:
The primary reason I think this theory is true is because Arnim Zola survived World War II and is somehow still alive to this day. There is also the fact that Arnim is responsible for Bucky's "death" in Captain America: The First Avenger. He could have easily gone back to the place where Bucky fell to take him back to some lab in order to revive him and then brainwash him to be a HYDRA assassin.
Rogue SHIELD agent:
Since Captain America, Black Widow, and Nick Fury seem to be fighting a rogue SHIELD led by whatever villain Robert Redford is portraying, it is also possible that Winter Soldier is working with them. In Avengers Tony Stark says that Nick Fury's "secrets have secrets." I believe that SHIELD may have secretly found Bucky and kept him a secret from Steve as they did whatever was necessary to save him. With a villainous Robert Redford leading SHIELD now leading SHIELD he could have chosen to unleash the Winter Soldier to destroy Captain America, Black Widow, and Nick Fury.
Brainwashed Soviet agent:
The only reason this seems to be a possibility is because there is the red star on Winter Soldier's arm. However, I really have to question if this is the case because I am wondering if the World War II fought in Captain America: The First Avenger can be considered the same as the World War II fought in real life? It just seems that there are too many differences with HYDRA being the main enemy in Europe, no mention of Italy or Japan, and the only Allies known are America, France, and Britain. In fact during the credits when old propaganda posters were being shown there was no Soviet flag in the poster where the guns representing different nations was shown, which is unlike the real life poster. This makes me wonder if the Soviet Union existed in the Marvel Cinematic Universe.
I also have to wonder about the Winter Soldier himself. Unlike in the comics, Bucky does not "die" from an exploding drone plane, but falls to his doom in a fight with HYDRA soldiers on a train. This begs the question of whether the arm is the only thing done to him, or if other measures were taken to save Bucky's life and that's why he covers up everything except his hair.
Whatever the case may be I am still looking forward to seeing the Winter Soldier next year.