Terran air units are (you guessed it) in the air. Most can attack both ground and air units, and are very powerful if you learn to master their skills. Some tactics of Terran air are to cloak, use powerful weapons, attack in groups, and the Science Vessel's multi-talented special abilities. Once you learn all about the air units you will most likely want to make hoards of air units, but remember...You should balance your units between ground and air forces.
You can view the Terran's Air Units really detailed information by clicking here, or by clicking on the specific unit's name.