Please don't apologize -- few people seem to understand this issue. At least you asked a question rather than assuming these were different names for the same kind of military unit!
What's the difference? It's quite similar to the difference between state troopers and the regular army. State troopers have local responsibilities and report to a state official; a soldier in the regular army goes wherever he's ordered. His chain of command is Federal rather than state, ultimately going through channels to the Secretary of War and the President.