U.S. Citizen - One who was born either within the territory of the United States or to U.S. citizen parents.
U.S. National - One who owes permanent allegiance to the United States.
Lawful Permanent Resident Alien - One who is legally accorded the privilege of residing permanently in the United States. They may be issued visas by the Department of State overseas or adjusted to permanent resident status by U.S. Citizenship and Immigration Services in the U.S.