Five primary component branches comprise the United States armed forces.
U.S. Air Force
U.S. Army
U.S. Marine Corps
U.S. Navy
U.S. Coast Guard
All are within the Department of Defense except the Coast Guard which is within the Department of Homeland Security. During time of war or national emergency, the Coast Guard functions as a specialized service under the Navy.
Each service has both an active and a part time Reserve component. Two services (Army, Air Force) also have state components operating under the National Guard Bureau.
Service Area of Operation
The services have operated in the three theaters (sea, land, air) and now there is a fourth (space).
If you want to fly, Each service branch has an aviation wing. The Air Force, Marines, Navy and Coast Guard all utilize both helicopters and fixed wing aircraft. The Army insignia appears primarily on helicopters, but it has the largest helicopter wing of all the services.
If you want to be a land-based service member, The obvious choices are the Army or Marine Corps. However, in keeping with their name, Marines operate off Navy craft at sea. The Air Force, Coast Guard, and Navy also have job specialties which can operate on land including both support functions and their elite Special Forces units.
If you like the sea, The Navy certainly fills that need, but there is also the Coast Guard which protects the nations ports and coastline.
If space intrigues you, Both the Air Force and Navy function in that area.
Service Missions
Air Force and Air Force Reserve: The primary mission of the USAF is to protect and defend the nation's interests in air, space, and cyberspace. The Air Force utilizes long and short range aircraft to transport troops and equipment and conduct air warfare. The Air Force is also identified with high technology, electronic warfare, and space. The Air Force has 4 job categories with a total of 144 individual enlisted job functions.
Air National Guard: The Air National Guard is a separate reserve component of the United States Air Force. The Air Guard is administered by the National Guard Bureau with dual federal and state missions. The Air Guard has total responsibility for air defense of the entire United States, and must maintain well-trained, well-equipped units available for prompt mobilization during war and national emergencies, including natural disasters and civil disturbances.
Army and Army Reserve: The largest of the military services, the Army is the land force that moves in to an area, secures it, and instills order and values before it leaves. It also guards U.S. installations and properties throughout the world. The Army includes the largest helicopter wing among all services for transport of troops and air support. The Army has 10 job categories with a total of almost 200 individual enlisted Military Occupational Specialties (jobs).
Army National Guard: The Army National Guard is an elite group of warriors who dedicate a portion of their time to the dual mission of protecting life and property in their home states and defending the United States and its interests worldwide. Each state has its own Guard, as required by the Constitution; in fact, it is the only branch of the military whose existence is actually required by the Constitution.
Coast Guard and Coast Guard Reserve: With its distinctive blend of military, humanitarian, and civilian law-enforcement capabilities, the Coast Guard’s mission is to protect the safety of the nation’s coasts, ports, and waterways. This includes maritime safety and rescues, maritime law enforcement, maritime mobility, national defense, and protection of natural resources. The Coast Guard has played a vital role in Homeland Security. The Coast Guard has 4 job categories with 18 individual enlisted Ratings (jobs) to select from.
Marine Corps and Marine Corps Reserve: The Marine Corps is known as the U.S.’ rapid-reaction force. Marines are trained to fight by sea and land, and take great pride in being elite warriors. They are usually involved in the most challenging situations. Marines also provide security for the nation's embassies overseas.
Navy and Navy Reserve: The Navy is recognized as the sea-going service as it protects the oceans around the world to create peace and stability, making the seas safe for travel and trade. While identified with the sea, the Navy has a large air wing and provides some of the most elite warriors (SEAL teams) around. The Navy is usually first on the scene in trouble spots to protect America's interests. You will get to travel if you join the Navy. There are 29 enlisted job categories and with three major operating groups (surface ships, submarines, aviation) these expand to over 1,000 potential enlisted and officer career specialties.
Visit the websites of each service to learn more and then start discussions with the nearest recruiter to determine if there is a fit for you.