The United States Air Force undertakes missions that fall within its six distinctive capabilities: air and space superiority, global attack, rapid global mobility, precision engagement, information superiority and agile combat support. These in turn are the result of the Air Force's three competencies, namely developing airmen, technology to war fighting and integrating operations, according the Air Force website. In practice, this means that the Air Force deals with missions relating to air and space defense, reconnaissance, logistics, nuclear deterrence, drone use and electronic warfare, among other areas.
The nature the Air Force's missions has changed over time. Long before the founding of the present day organization, the U.S. military used aircraft primarily for scouting out the battlefield. This included the use of balloons during the Spanish and Civil Wars. However, while reconnaissance has remained a central focus of military air operations, the two World Wars added combat and eventually bombing to the list. More recently, deliverance of nuclear payloads, drone use and electronic warfare have become important aspects of military air operations.
The USAF itself is a relatively new branch of the military. It can trace its roots back to the Aeronautical Division of the U.S. Signal Corps and successor organizations like the U.S. Army Air Corp. It existed under several names under the direction of the U.S. Army initially, although by World War II it was de facto independent. It wasn't until 1947 that the Air Force gained its present name and became a fully independent branch of the military.