Please read camber as a prerequisite. The caster concept is based on camber, but involves different conditions.
Caster acts as varying degrees of camber based directly on the steering angle. Motorcycles tend to have a lot of caster. It is used instead of static camber so the contact patch isn't sacrificed when the vehicle is not cornering. Because it is a direct result of the steering angle, however, forces act against the steering components and driver more. This effect is generally small, but noticable in high-grip vehicles without power steering.
Caster's Effect on Tire Wear
Because caster is only giving the wheel an angle when cornering, the wear is less than that of camber. However, like camber, too much of it can produce uneven tire wear.