The District of Columbia is not a state, it is a federal district. When the Constitution of the United States was adopted in 1787, what is now the District of Columbia was a part of the state of Maryland. In 1791, the District was ceded to the federal government for the purpose of becoming the nation's capital, a district that was to be governed by Congress.
In 1973, the District of Columbia Home Rule Act provided the city with more control of local affairs, including the election of a mayor and a 13-member city council. Residents of the District of Columbia lack full democratic representation in the U. S. Senate or the U.S. House of Representatives. Representation in Congress is limited to a non-voting delegate to the House of Representatives and a shadow Senator. In recent years, District residents have been seeking Statehood to gain full voting rights. They have not yet been successful. Read more about DC Voting Rights.
For more information, read DC Government 101 - Things to Know About DC Laws, Agencies and More