Caster is a bit harder to conceptualize, but it’s defined as the angle created by the steerings pivot point from the front to back of the vehicle. if you imagine your local supermarket's shopping trolleys, the caster wheel almost always returns to the centre when push forward, this effect is the same as what's happening on your vehicle. Caster is positive if the line is angled forward, and negative if backward.
Typically, positive caster will make the vehicle more stable at high speeds, and will increase tire lean when cornering. This can also increase steering effort as well.
Most road vehicles have what is called cross-caster. Cross castered vehicles have slightly different caster and camber, which cause it to drift slightly to the right while rolling. This is a safety feature so that un-manned vehicles or drivers who lose steering control will drift toward the side of the road instead of into oncoming traffic.