Brock Rumlow was a young gangbanger when circumstances landed him under the tutelage of the Taskmaster at one of his "academies" in which he trained up and coming criminals so he could farm them out to the highest bidder. Rumlow was such an excellent student, however, he eventually became one of the instructors, known for his brutality.
Upon leaving the school, Rumlow became a mercenary, working in Algeria for Albert Malik, named the Red Skull by the Soviets, and the man responsible for the murder of Peter Parker's parents, Mary and Richard. While working for Malik, Rumlow was sent to Switzerland on an invasive mission of Dr. Arnim Zola's lab, where the original Nazi Red Skull happened to be. While Rumlow's entire team was killed in the assault, he survived and ultimately came to work for the the Skull who gave him his new name: Crossbones. Despite a hiccup here and there, Crossbones stayed fiercely loyal to the Skull, especially during the events surrounding Ed Brubaker's Winter Soldier story line upon which the upcoming film is based.
Crossbones has had many run-ins with Captain America and his various allies, including Bucky and The Falcon over the years, including a couple that left him critically wounded and imprisoned. Despite not being a super-powered villain, Crossbones training by Taskmaster combined with physicality and athleticism make him one of the world's most impressive hand-to-hand combatants. He has gone rounds with Cap, Bucky and The Black Widow and has more than held his own on most occasions and, at one point, brutally beating a distracted Captain America.
Captain America: The Winter Solider looks to give Rumlow a slightly different origin. The trailers and footage seem to indicate that, at least early on, Rumlow will be an Agent of S.H.I.E.L.D. under the command of Robert Redford's Alexander Pierce, who many, including myself, believe to be the film's true villain and possibly the new incarnation of the Red Skull. Early in the trailer, Rumlow is seen aboard an aircraft with Captain America before the latter jumps out of the plane, parachute free.
Later in the trailer, we see that Rumlow, though working for S.H.I.E.L.D., is not working on the same side as Rogers, as he and several other agents attack Cap in an elevator. That attack, of course, goes poorly, leaving Rumlow and the others, on the floor while Cap makes his exit.
While none of the footage shows Rumlow in his infamous Crossbones mask,Grillo, in a recent interview with Crave Online, teased fans about the future of Crossbones in the MCU, saying only that if the fans responded well to the character that it was likely we'd see more of him in future films.
Crossbones has a fairly defined role within the comic book arc of the Winter Soldier, however, Marvel is becoming pretty famous for not strictly following predefined comic book origins and histories so fans of the classic story line may be in for a few surprises as to just how Crossbones will be portrayed on screen. Safe to say, however, Marvel isn't going to make Rumlow a good guy and, at some point, he'll probably have a good reason to put on the iconic mask.