"The filibuster is widely viewed as one of the Senate's most characteristic procedural features. Filibustering includes any use of dilatory or obstructive tactics to block a measure by preventing it from coming to a vote. The possibility of filibusters exists because Senate rules place few limits on Senators' rights and opportunities in the legislative process. In particular, a Senator who seeks recognition usually has a right to the floor if no other Senator is speaking, and then may speak for as long as he or she wishes. Also, there is no motion by which a simple majority of the Senate can stop a debate and allow the Senate to vote in favor of an amendment, a bill or resolution, or any other debatable question. Almost every bill, indeed, is potentially subject to two filibusters before the Senate votes on whether to pass it: first, a filibuster on a motion to proceed to the bill's consideration; and second, after the Senate agrees to this motion, a filibuster on the bill itself. Senate Rule XXII, however, known as the 'cloture rule,' enables Senators to end a filibuster on any debatable matter the Senate is considering. Sixteen Senators initiate this process by presenting a motion to end the debate. The Senate does not vote on this cloture motion until the second day of session after the motion is made. Then, for most matters, it requires the votes of at least three-fifths of all Senators (normally 60 votes) to invoke cloture."
CRS Report for Congress, RL30360