http://www.senate.gov/reference/resources/pdf/RL30360.pdffilibusters 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 after the motion is made. Then it usually requires the votes of at least three-fifths of all Senators (normally 60 votes) to invoke cloture. Invoking cloture on a proposal to amend the Senate’s standing rules requires the support of two-thirds of the Senators present and voting.
The primary effect of invoking cloture on a question is to impose a maximum
of 30 additional hours for considering that question. This 30-hour period for
consideration encompasses all time consumed by rollcall votes, quorum calls, and
other actions, as well as the time used for debate. During this 30-hour period, each Senator may speak for no more than one hour apiece (although several Senators can have additional time yielded to them). Under cloture, the only amendments that Senators can offer are amendments that are germane and that were submitted in writing before the cloture vote took place. The presiding officer also enjoys certain additional powers under cloture: for example, to count to determine whether a quorum is present, and to rule amendments, motions, and other actions out of order on the grounds that they are dilatory.