A Bill of Rights is a list of things that a government may not do to its subjects or citizens.  The English have one, dating back to 1689.  The American states usually included one with their new constitutions as soon as they became independent.  When the Constitution was first submitted to the states for ratification, it did not include a bill of rights.  The Federalists, who argued in favor of the new Constitution, argued that there was no need for a Federal Bill of Rights: the Federal Government, they argued, was a government of strictly limited powers.  There was no need of further protection beyond its structure.  Those suspicious of the new constitution were hardly reassured by this argument.  So the Federalist made a promise: ratify the Constitution, and we will amend it to include a Bill of Rights.  True to their word, it was one of the first acts of the new Congress: out of a number of amendments suggest, ten were proposed and ratified.

So today when we use the term Bill of Rights, we usually mean the first ten amendments to the Constitution of the United States.