A Member of Congress (MC) is a person who has been appointed or elected and inducted into some official body called congress, typically to represent a particular constituency in a legislature. Member of Parliament (MP) is an equivalent term in other jurisdictions.
In the United States, the federal legislature is called the Congress. Members of the Senate, the upper house of Congress, are called "Senators" and are addressed "Senator [Name]". The President and President Pro Tempore of the Senate are exceptions to this rule. They are addressed "Mister President" or "Madame President", when serving in that capacity. Otherwise, the President of the Senate holds that office ex officio as Vice President of the United States, so he is addressed "Vice President" outside of the Senate.
Members of the lower house, the House of Representatives are called "Representatives or Members of Congress" but are addressed "Congressman [Name]" or "Congresswoman" - except for the Speaker of the House of Representatives, who is called "Speaker [Name]" and is formally addressed "Mister Speaker" or "Madame Speaker".