For years, Captain America fought a terrorist organization called Hydra, whose motto was "Hail HYDRA! Immortal HYDRA! We shall never be destroyed! Cut off one limb and two more shall take its place!" Apparently, Cap learned from their example, because even though one Cap was assassinated on the steps to a courthouse following the events of Marvel's Civil War, there are now two Captain Americas running around*, trying to save the country. How did that happen?
Well, it all started with Steve Rogers' assassination. Or, rather, faux-assassination. What at first seemed like the original Captain America's death was actually part of a plan on behalf of his former WWII nemesis, the Red Skull, to ruin the USA and place his mind within the body of his most hated enemy. He hadn't figured on the interference of Cap's former sidekick (turned brainwashed cyborg assassin, turned deprogrammed troubled vigilante), Bucky Barnes, who not only saved the country but did so while taking on the name of Captain America for himself.
That didn't mean that the Skull had abandoned his plans to take the original Captain America's body for himself, however. But when he tried putting that particular plan in motion, he was foiled not only by the new Captain America, but also a group of superheroes who had independently realized that Steve Rogers wasn't dead, but lost inside his own time period, being thrown from traumatic experience to traumatic experience in an attempt to weaken his will. Rogers was returned to the present physically unchanged but with a new perspective on his life as a result of his time travel. At the start of Siege, both Steve Rogers and Bucky Barnes are active as Captain America.
* - There is actually a third Captain America running around, a brainwashed racist who believes that he is the original. But, for the most part, he doesn't count.