An air force is the branch of a country's military which fights in the air. The air force is made up of aircraft such as fighters, bombers, helicopters and transport aircraft. When a navy has its own air force it is called Naval Aviation.
Air forces began in the early 20th century, usually as part of the army. Most countries separated their air forces in the 1930s and 1940s.
Aircraft in an air force sometimes fly in a formation. Formations are when the aircraft fly in a pattern. Air forces is part of the military.