Army: The Army is the oldest of the branches, followed closely by the Navy and the Marines. It is also the largest of the branches. The main role of the Army is to conduct land-based military operations, but it also provides manpower for humanitarian missions (e.g., building schools in war-torn regions and responding to Ebola in Africa). The Army uses many armored and weapons-equipped vehicles during their missions, as well as a variety of helicopters.
Air Force: The Air Force is the youngest branch of the military; until 1947, it was a part of the Army. The Air Force conducts air-based missions (e.g., using firepower or conducting rescue missions to support ground troops). Members of the Air Force are not all pilots, but also parachuters, air traffic controllers, and experts in reconnaissance. Outer space and cyberspace (for the purposes of intelligence and surveillance) also fall under the jurisdiction of the Air Force.
Coast Guard: The Coast Guard is the smallest of the branches. Its job is to conduct water-based missions around U.S. borders to protect people using the waterways, provide search and rescue, prevent illegal immigration and the transport of drugs into the country, and protect the marine environment. In times of war, the Coast Guard falls under the jurisdiction of the Navy.
Marines: The Marines work closely with the Navy, and thus are mainly water-based. However, Marines conduct both land- and water-based missions depending on where they are needed. Their main role is to be a quick response force in times of crisis; troops can deploy in a matter of days, if need be. Missions may involve providing humanitarian relief following a natural disaster or evacuating U.S. citizens from volatile areas. Marines also provide on-land support to Naval missions. Each Marine receives training as a rifleman regardless of their specialty to ensure that all troops are capable during a mission.
Navy: The Navy conducts mainly water-based missions, but also includes the Navy SEALs, a special operations force within the Navy who perform sea, air, and land missions. With the largest fleet of aircraft carriers in the world and over 250 combat ships and submarines, the Navy has a global presence and works with the Marines to provide quick responses to immediate threats. The Navy also operates fighter jets and logistics aircraft that protect aircraft carriers and provide intelligence.