A fighter is an air unit whose purpose is to destroy other air units. Each faction has a tech 1 Interceptor and a tech 3 Air Superiority Fighter. The Cybran, UEF, and Seraphim also have tech 2 fighter/bombers that have anti-air weaponry. The Aeon are the only faction to have a dedicated tech 2 fighter in the Aeon T2 Combat Fighter: Swift Wind.
The Anti-air strategy article describes the use of fighters as part of a larger anti-air strategy.