The two parts of the US Congress are The United States Senate and the United States House of Representatives together constitute that which is known as Congress, which is The Legislative Branch.
Senate - Upper House - currently 100 members, 2 from each state, serving 6-year terms with one-third of the members coming up for election during each two-year election cycle. The Senate is based on equal representation for all states. All states have two senators. The seats in the Senate were originally appointed by the State Legislatures but, with the 17th Amendment, were opened to popular vote.
House of Representatives - Lower House - currently 435 members, representing approximately equal numbers of citizens, serving 2-year terms with all members coming up for election each two-year election cycle. The House is based on population and the number of representatives is based on how many people are in your state. The seats in the House of Representatives were always filled by popular vote.