The United States Marines are an Amphibious force by nature but contain several elements that make them *almost* fully independant from all the other branches, example (Aviation Wing both Fixed and Rotorary Wing, Infantry etc etc) as well as a Rapid Deployment force known as a Marine Expiditionary Unit.
The Air Force deals mostly in aviation including ballastic missiles, test flights..
The Army has everything except Jet Aircraft, mostly an occupying force with a shitload of people.
Army Rangers are idiots that think they are all that and a bang of potato chips with their super dooper jump school but in reality are no more superior than that of a regular Marine Infantrymen.
The Coast Guard guards the coast and rescues people at high seas, drug enforcement.
The way I see it is, the Army Reserve is a federal force, whereas the national guard can be mobalized on a state level, the Governer of a state and have the national guard help at a moments notice, where the army reserve takes an act of congress and blow job to a high ranking general to get them to follows orders based on a state level.