Caster is an angle that most do not understand. Something you should understand about caster is that it is not a live angle. As we know, that means it does not update itself in real time on the alignment machine and it means that it does not cause tire wear. It does, however, cause a alignment "pull".
Caster is responsible for proper steering and also for keeping the vehicle in a straight line. A caster difference from side to side will cause a pull to the LESSER measurement. For example, a reading of +1.0 on the left side and -2.0 on the right side, then the car will pull to the right. A small positive caster is desired, so that the wheel will be easier to return to the center-line after a turn.
Extremely positive caster adjustments will cause increased effort in steering, excessive bump steer, and the wheel will want to remain straight. Extremely negative caster adjustments will cause the steering to feel easier but the car will have a harder time keeping in a straight line. Cars designed for performance and handling typically have a higher caster rating for straight line handling improvement, and a huge semi will typically have a lesser caster rating for easier steering.
When viewed from the side, a positive caster will appear as the strut will be leaning back toward the seats of the vehicle. A negative caster will seem like the strut is leaning toward the front fender. Most vehicles have a positive caster rating, from about .25 to 3.0.
On many vehicles, caster is not adjustable. Some allow adjustment via camber bolts in the upper control arms. On vehicles that do not allow adjustment, the only way to current the problem is to fix the broken or bent suspension parts.