From Academic Kids
A Congressman or Congresswoman (generically, Congressperson) is a politician who is a member of a Congress. In countries with a parliament rather than a congress, "MP" (Member of Parliament) is used instead.
In the United States, a Representative usually refers to a member of the country's House of Representatives. A Senator is a member of the United States Senate. A Congressman, or Congressperson refers to both, members of the United States Congress. But more often the term, or title, of "Congressman / Congresswoman" is usually used in conversation to address a member of the House, for example "Hello Congressman Jones."
Note: American political scientists, in a further effort to clarify the issue, now use the term Member of Congress (MC), to refer to both members of the Senate and the House, and stick to Senator and Representative when referring to members of either specific body. These terms - Member of Congress, Senator, Representative - all have the advantage of being gender-neutral, reducing the number of variant terms needed. The term also brings the U.S. into line with the rest of the world's parliamentary bodies, reducing confusion abroad.