The Bill of Rights makes up the first ten amendments to the Constitution and includes such things as the right to free speech and religion (1), as well as the right to bear arms (2) and protection from cruel and unusual punishment by the government (8).
As an amendment is a written change to the Constitution, these were added on to the original document that was adopted in 1789. The Bill of Rights was adopted all at once two years later, in 1791. This made the Anti-federalists breathe a little easier, because it gave them written guarantees to prevent a dictatorship and assure the rights they felt they had fought the Revolution for.
December 15th, 1791 was when the Bill was ratified into the Constitution as the first ten amendments.