The United States Secret Service is a branch of Homeland Security, with two distinct missions. It serves as the investigative arm of the U.S. Treasury, investigating counterfeiting and other threats to the national finances.
Its other duty, and the one for which it is better known, is protective services. Protection is provided for sitting presidents and for former presidents, and other major government officials, including viable candidates for a forthcoming presidential election.
Originally, the Secret Service had wider roles, but many of these have been taken over by other federal agencies.