Airplanes are perfect for aerial reconnaissance, bombing enemy units and for air raid defenses over your provinces. In order for airplanes to take-off, it is necessary to build an aerodrome, but an aerodrome is not required to produce aircraft.
Airplanes allow you to do aerial reconnaissance. During this patrol, the airplanes are collecting information about the strength and formation of enemy units. In addition, patrolling airplanes attack enemy airplanes which execute orders in the range of your patrol. Airplanes that are just flying through the radius won't be attacked.
This patrol area is visualized by a circle indicator. Airplanes can patrol within the plane's operating range.
The Fighter is weaker, but faster than the bomber.