"The filibuster is a U. S. Senate practice whereby a single Senator, or his minority party, can block full Senate consideration of a bill or nomination by extending debate on the proposal indefinitely. The resulting "filibuster" can ordinarily be stopped only by a "cloture" vote, which requires 60 of the 100 Senators (a supermajority) to vote to end debate, and bring the bill or nomination to a final vote.

The filibuster, obviously, plays a crucial role whenever only 50 to 59 Senators support a given bill or nomination. It can ensure that the proposal is not approved, despite the fact that a majority of the Senate (or half the Senate, plus the Vice President, who breaks ties) would approve the measure if a vote were taken.

This right to endless debate evolved in the Senate over the eighteenth and nineteenth centuries. It is now embodied in the Senate's own formal rules - particularly Rule XXII, which guarantees unlimited discussion absent cloture.

Filibusters of old required the filibustering party to actually stand and hold the Senate floor by speaking continuously, in order to delay a final vote, as depicted in Frank Capra's famous movie "Mr. Smith Goes to Washington," which featured Jimmy Stewart as a young and idealistic Senate newcomer. Today, by contrast, a minority party can indefinitely put off a vote on a bill or nomination by simply indicating to Senate leaders that this indefinite delay is desired - unless there are 60 Senators in favor of cloture."

http://www.yuricareport.com/Law%20&%20Legal/Senate%20Rules%20on%20Filibuster.html