"Air Force Two" is the designation given to any Air Force-operated aircraft that the Vice President of the United States is travelling aboard. If the President of the United States travels with the Vice President, the "Air Force One" designation is used.
"Air Force Two" is most commonly affiliated with the C-32 aircraft, which is a Boeing 757-200 modified for transportation of government personnel, but does not refer to one specific plane or plane model. When the Vice President rides aboard a VC-25A, the aircraft usually used to transport the President, that plane is called Air Force Two by all FAA and military operatives. Similar terminology applies to the use of aircraft from other military branches as well, such as Marine Corp. helicopters being called "Marine Two" when the Vice President is on board.