Marines aren't just like Army soldiers with better uniforms. They are different. If you want to understand why they're different, a good place to start is the National Museum of the Marine Corps in Quantico, Virginia. It opened in November, 2006, and it contains a number of well-done exhibits about Marine Corps training, the Marine style of fighting (in teams that closely integrate infantry, artillery, and air, with riflemen who are trained to take the initiative, even if they're extremely junior in rank), and their major deployments since they were founded as the Continental Marines in 1775. Not surprisingly, the emphasis is on World War II (where the Marines did the vast majority of the fighting in Nimitz's island hopping campaign in the Central Pacific), the Korean War, the Vietnam War, and the Iraq/Afghanistan wars. There are plenty of artifacts and film shorts, but there is an emphasis on reading -- don't bother to come here unless you're prepared to read a lot. And don't come here if you're unwilling to spend quite a lot of time going through it -- it's possible to spend multiple hours in each gallery and still not get everything in them that there is to get.
The picture above is of the facade of the museum, and you may have noticed that it bears a certain resemblance to that famous picture of the Marines raising the American flag on Mt. Suribachi on Iwo Jima in 1945. Obviously, the resemblance is intentional (and, of course, the museum has both of the flags that those marines raised, although only one is displayed at a time). Admission to the museum is free, and it deserves to be on the itinerary of anyone who lives or visits the Washington, DC area.