Filibusters and Cloture in the Senate

"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 that Senator 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 itself to vote in favor of an amendment, a bill or resolution, or most other debatable questions. Most bills, indeed, are potentially subject to at least two filibusters before the Senate votes on final passage: 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..." 
Filibusters