Unlike the Marines, which are a corps of the Navy (which means they still get their checks from the Navy, that's right Greg, I said it, lol), the Army is it's own branch of the U.S. Military, and encompasses a wide variety of equipment and jobs. They have their own aircraft (in fact, the Air Force was originally part of the Army), and are capable of completing their missions independently from any other branch of the service.
When it comes to Marines, there are only two things you really need to know;
1) Never, EVER call them "soldier".
2) "In the language of death, the Marines are poets".
There are several really great Hubs on both branches out there, check over to the right in the "Related Hubs" section for more information.