A senator is a congressman, a congressman isn't necessarily a senator. Congress is made up of the Senate and the House of Representatives, and both senators and representatives of the House are referred to as congressmen. A senator can always be referred to as a congressman, but a congressman is not necessarily a senator in the event they are a representative.
The upper chamber of the U.S. Congress is the Senate, and the lower chamber is the House of Representatives. There are 100 sitting senators, and each of the 50 states has two senators. Depending on the size of the state in question, one senator can represent between 1 million and 37 million people. Senators serve staggered terms of 6 years.
There are 435 house representatives for each of the nation's 435 congressional districts. Each congressional district has a little over 700,000 people, and each district elects a representative to serve terms of 2 years in length. In general, the House and the Senate have an equal amount of legislative authority. Congress serves an important part in the operation of national defense. Congress has the exclusive power to make declaration of war, to augment the armed forces and to dictate rules for the military.