The US Constitution was written in 1787 and ratified by the original 13 states, establishing it as the highest law of the land. It outlines how the US government works through its three branches - legislative, executive, and judicial. The Constitution can be amended, with the Bill of Rights being the first 10 amendments ratified in 1791 to protect citizens' rights. A key example is how the 13th, 14th, and 15th Amendments ended slavery and guaranteed rights of citizenship and suffrage for African Americans.