A Member of Congress ( MOC ) is a person who has been appointed or elected and inducted into an official body called a congress, typically to represent a particular constituency in a legislature. Member of Parliament (MP) is an equivalent term in other jurisdictions.
In referring to a lawmaker in their capacity of serving in Congress the term Member of Congress is used less often than other terms in the United States. This is because in the United States the word Congress is used as a descriptive term for the collective body of legislators, from both houses of its bicameral federal legislature: the Senate and the House of Representatives. For this reason, and in order to distinguish who is a member of which house, a member of the Senate is typically referred to as Senator (followed by "name" from "state" ), and a member of the House of Representatives is usually referred to as Congressman or Congresswoman (followed by "name" from the "number" district of "state" ), or Representative ( "name" from the "number" district of "state" ). Although Senators are members of Congress, they are not normally referred to and addressed as "Congressmen" or "Congresswomen" or "Congresspeople".
Members of Congress in both houses are elected by direct popular vote. Senators are elected via a statewide vote and representatives by voters in each congressional district. Congressional districts are apportioned to the states, once every ten years, based on population figures from the most recent nationwide census. Each of the 435 members of the House of Representatives is elected to serve a two-year term representing the people of their district. Each state, regardless of its size, has at least one congressman or congresswoman. Each of the 100 members of the Senate is elected to serve a six-year term representing the people of their state. Each state, regardless of its size, has two senators. Senatorial terms are staggered, so every two years approximately one-third of the Senate is up for election. Each staggered group of one-third of the senators is called a 'class'. No state has both its senators in the same class. 
History of the United States Congress
The United States Congress was created in Article I of the Constitution, where the Founding Fathers laid out the limitations and powers of Congress. Article I grants Congress legislative power and lists the enumerated powers and allows Congress to make laws that are necessary and proper to carry out the enumerated powers. It specifies the election and composition of the House of Representatives and Senate and the qualifications necessary to serve in each chamber.
The Seventeenth Amendment changed how senators were elected. Originally, senators were elected by state legislatures. The Seventeenth Amendment changed this to senators being elected directly by popular vote.