What is a Filibuster?
A filibuster is a tactic in the legislative process sometimes used in the U.S. Senate by opponents of a bill to block its passage. A Senator, once granted permission to speak by the presiding officer, may continue to speak indefinitely in an effort to delay or prevent a final vote on the bill. To halt the filibuster, the Senate must pass a "cloture" resolution by a three-fifths majority (60 votes). Filibusters are not allowed in the House of Representatives because House rules limit the time allowed for debate on bills.
Bills considered in the Senate under the "budget reconciliation" process are not subject to filibusters.
Note: On November 21, 2013, the Senate voted to require a simple majority vote of 51 Senators to pass cloture motions ending filibusters on presidential nominations for Cabinet secretary posts and lower federal court judgeships only. See: Senate Democrats Take the ‘Nuclear Option’
Also See: Five Longest Filibusters in US History