My understanding is that the National Guard is, when activated, just another active duty arm of whatever service it is a part of.
National Guard units can be Federalized through a formal and limited process - i.e. brought under direct Federal command - but this is rarely done (e.g. Vietnam, and a few went to Iraq).
While remaining under the command of the State Governor (the usual state), they are exempt from Posse Comitatus, and can enforce laws domestically.