There are different types of military forces. In the United States, we have:

• the Army, which fights battles on the ground.
• the Navy, which operates in the ocean
• the Marines, who are trained to fight between the ocean and the land.
• the Air Force, which does attacks from the air.

But when people are speaking in general about all of these groups, they usually call them "the military" (which is the correct term) or "the army" (which is incorrect but often used as a general term for all military branches).