The first ten amendments to the U.S. Constitution are called the Bill of Rights, and they were ratified on December 15, 1791. The Bill of Rights protects rights such as freedom of speech, freedom of religion, the right to bear arms, the right to assemble, the right to fair trial, and the freedom of the press. It protects against things like unwarranted searches, cruel and unusual punishment, and excessive fines. Including the Bill of Rights, 27 amendments to the U.S. constitution have been ratified.
The U.S. Constitution delineates two ways in which an amendment may be proposed. One way is for a bill to pass both the Senate and the House of Representatives by a two-thirds majority. If the bill passes both houses of Congress, it goes to the state governments, where it must be ratified by at least three fourths of the states. The second way is for a Constitutional Convention to be called for by two thirds of the states' congresses. The Constitutional Convention would then propose one or more amendments which would be sent to the states. The amendments would then need to be approved by three fourths of the states. Such a Constitutional Convention has not yet been called for.
After an amendment has been ratified by Congress there are two ways that the amendment may be ratified by the states--either by a state convention or by state legislature. If the bill must be passed by a state convention, this will be stated within the text of the amendment. Otherwise the pill must be passed by state legislatures. To date only the 21^st amendment has been ratified by a state convention.
In addition to the 27 ratified amendments to the U.S. Constitution, another six have been ratified by Congress and not by the state legislatures. Of these, four are still technically pending and two have expired (one of these expirations is debated). The 27^th amendment was proposed in 1789, and was not enacted until 1992--making it the longest-pending amendment to have been ratified. Usually a time limit is described within which the states must ratify the amendments.