Unanimous Consent
From its beginning, the Senate has transacted much of its business by unanimous consent. The Senate's small size, few rules, and informality encouraged the rise of this practice. A single objection ("I object") blocks a unanimous consent request. Even several of the Senate's early rules incorporated unanimous consent provisions to speed the Senate's routine business.
Two types of unanimous consent are prevalent in today's Senate. Simple unanimous consent requests deal with noncontroversial matters, such as senators asking unanimous consent to dispense with the reading of amendments. Complex unanimous consent agreements establish a tailor-made procedure for considering virtually any kind of business that the Senate takes up. They are commonly brokered by the parties' floor leaders and managers. Two fundamental objectives of these accords are to limit debate and to structure the amendment process. As two Senate parliamentarians wrote in the Senate's volume of precedents: "Whereas Senate Rules permit virtually unlimited debate, and very few restrictions on the right to offer amendments, these [unanimous consent] agreements usually limit debate and the right of Senators to offer amendments."
http://www.senate.gov/legislative/common/briefing/Senate_legislative_process.htm
