When the Constitution was first written, it outlined the basic form of government for the United States. However, it received a lot of criticism for one specific reason: it did not have a bill of rights. Such a bill would outline and protect the freedoms of individuals.
At that time in history (the late 1700s), the U.S. had just fought a very difficult war to win freedom for its people. Even though this Constitution showed how it would limit and check the power of the government, many of the citizens didn't trust any government to protect the freedom of its citizens. Several of the states said they wouldn't ratify (approve) the new Constitution unless a bill of rights was added. Finally, under the condition that an amendment would be added containing a bill of rights, the Constitution was ratified by every state.
In 1789, James Madison wrote 12 amendments. Ten of these amendments were ratified and became our Bill of Rights.