Congress, including both the House of Representatives and the Senate, falls under the legislative branch of the U.S. government. Article I of the Constitution spells out the duties of both the House and the Senate, both individually and as a whole.
Congress is the only body of the government that can make or change laws. In addition to lawmaking responsibilities, only Congress has the authority to declare war. It also functions to confirm or deny certain presidential nominations as well as to ratify treaties. When necessary, the House of Representatives may vote to impeach officials, and the Senate tries them.