Who is a Member of Congress?

A Member of Congress is a person serving in the House of Representatives or the Senate. A Member of the House of Representatives is referred to as Representative or Congressman or Congresswoman, and a Member of the Senate is referred to as Senator.

Who is a Delegate? A Resident Commissioner?

The Office of Delegate was established by ordinance from the Continental Congress (1774-1789) and confirmed by a law of Congress. From the beginning of the Republic, accordingly, the House has admitted delegates from Territories or districts organized by law. Congress created the post of resident commissioner in 1904 to apply to Puerto Rico and the Philippines. Since 1946, only Puerto Rico has had a resident commissioner.
Delegates and the resident commissioner may participate in House debate but they are not permitted to vote on the floor. Delegates and resident commissioners have no vote in the Committee of the Whole. All serve on committees of the House and possess powers and privileges equal to other Members in committee. Currently, there is one delegate for each of the following: the District of Columbia, the Virgin Islands, Guam and America Samoa; as well as a resident commissioner from Puerto Rico. The formal duties of the delegates and the resident commissioner are identical, however, a delegate serves a two-year term while a resident commissioner serves a four-year term.

What is the proper form for addressing a letter to a Member of Congress?

Acceptable forms of address for Members of the House include "The Honorable" and "Representative." Correspondence may be addressed as follows:

The Honorable J.Q. Smith
US House of Representatives
Washington, DC 20515

or

Representative J.Q. Smith
US House of Representatives
Washington, DC 20515