The Vice-President of the United States is a Constitutional role, with the primary function of being the next in line to the presidency, should the President be killed or otherwise unable to fulfill his duty.
The Vice President is also the President of the Senate, able to break ties should the Senate vote a tie.
The current Vice President is Joe Biden.
Because the Vice President may have to become the President, he or she must be qualified to be President, meaning they be a natural born U.S. citizen; they must be 35 years old; and they must have lived in the U.S. for at least 14 (consecutive?) years.
Election of the Vice President
Originally, the Vice President was the person who received the 2nd most number of votes in an Presidential Election. This system broke down, however, in 1796, when Federalist John Adams won the presidential election and Democratic-Republican Thomas Jefferson came in second, leading to an executive split between the two parties. There were more problems in 1800 as the Democratic-Republicans failed in trying to engineer the second place finisher, forcing the House of Representatives to choose between Thomas Jefferson and fellow Democratic-Republican Aaron Burr.
In 1804, the Twelth Amendment was adopted to solve this problem, creating separate ballots for President and Vice President.