The US Army and the US Marines are two very different services, with unique histories and goals, although there is some overlap in their capabilities in combat. The mission goals for the US Army and the US Marines are different, and they accomplish their work in different ways. While people might be tempted to lump the two together since they both form parts of ground-based invading forces, most soldiers and marines would resent implication that the two forces are indistinguishable.
The United States has four armed forces under the direction of the Department of Defense (DoD): the US Army, US Air Force, US Navy, and US Marine Corps. The Coast Guard, the other armed force of the United States, is technically under the wing of the Department of Homeland Security, although it operates under the DoD during times of war. Each branch of the armed services in America performs a vital role for national security.
The US Marines are a highly mobile amphibious attack force. Marines are trained to attack from the water and establish a beach head, an area of control on foreign soil. After the Marines take territory, other armed forces, such as the Army, move in to maintain control, while the Marines move on. Marines are mobile, lightweight, and very rapid. One might compare the Marines to the head of a spear, wedging in to get a foothold and racing ahead once the land has been secured.
In addition to acting as a lightweight attack force from the ocean, Marines are also perfectly capable of taking territory on land. Marines are trained for rapid deployment, and are often the first US military personnel on site. Marines also guard American embassies overseas, providing embassy security and safety. In volatile areas, being a Marine embassy guard is a very risky job.
The US Army, on the other hand, is the primary ground-based military force. As such, the US Army captures and holds territory with the use of infantry, aircraft, and an extensive support staff. The US Army is in the thick of battle, and is a substantially larger armed force than the Marines.
Both the US Army and the US Marines have reserve troops, which can be activated in times of need. The Army reserves are significantly larger, however. Unlike the Marines, the US Army also has extensive support staff, including medical personnel. The Marine Corps relies on the Navy for many support services, keeping the service small and efficient.