A more thorough explanation:

The United States Army is the branch of the United States armed forces that is responsible for land combat and land operations. It includes supporting air and water transport services such as the Army Air Corps. The Army is made up of the Regular Army (the standing force), the Army Reserve, and the Army National Guard when in active federal service, such as in times of war or other national emergencies. The U.S. Department of the Army is responsible for overseeing the United States Army.

For example, during times of war, the United States Army may be called upon to engage in combat on land. They may also be responsible for transporting troops and supplies to different locations using air and water transport services. The Army National Guard may also be activated to assist in emergency situations, such as natural disasters.