The United States Army is the ground branch and the largest of the military branches in the country. While the Army defends the land, the Navy defends the seas.
The United States Navy is the main supporter of the Air Force. It features aircraft carriers to help provide runways and aircraft to the other branch. Soldiers can enlist full-time or in the reserves, just like in the Army.
The Army was established in 1775 and has active duty soldiers and two reserves, the National Guard and the Army Reserves. What makes the two reserve branches different is that the National Guard is controlled by the states and the Army Reserves falls under federal control. The majority of soldiers in the Army are on the ground.
Other branches of the military serve their own purposes. The Coast Guard, for example, is the smallest branch of the military and like the Navy, the Coast Guard performs most of its duties on the water. The Navy has the power to call the Coast Guard to help with its missions, should the need arise.
The Air Force supports ground missions by providing the other military branches with air support. The Air Force didn't become a separate branch of the military until 1947.