Attack aircraft consist of all manner of aircraft which are geared primarily towards providing tactica support for ground units. Normally, fighter aircraft can target land units and so do bombers too, but their ability to damage ground targets is limited — fighters are worthless against buildings, while heavy bombers have an attack penalty versus units.
So as a result, attack aircraft are useful in defending against enemy units. They are the best units to be used to target units, especially mobile AA units and tanks, and come in two variations:
attack fighters, which are armed with autocannons, and
light bombers, which are useful against non-infantry units.