The United States Congress has two chambers: the House of Representatives (ften referred to simply as "the House") and the Senate. The House does not have staggered terms: every single member of the House of Representatives is elected to a two-year term, and all of the seats in the House are voted on at the same time.
The Senate, however, does have staggered terms. Senators are elected to six-year terms, but only a third of the seats in the Senate are up for election in a given election year. Federal election years are held on even years (2010, 2012, 2014, etc.), and on any given year only about a third of Senators will face reelection. This framers of the Constitution designed the Senate this way to make sure there was some continuity from one Congress to the next.
