Seen from the side of the vehicle, the steering axis centerline is normally tilted from the vertical.
Caster is the angle formed by this line, and a line drawn vertically through the center of the wheel.
Backward tilt from the vertical is positive caster.
Forward tilt is negative caster.
When a vehicle has positive caster, a line drawn through the steering axis centerline meets the road surface, ahead of the centerline of the wheel. The tire contact point is behind the steering axis.
When the wheel is turned to the right, the tire contact point is moved to the left of the direction of travel. And similarly for turning to the left.
In forward motion, this generates a self-centering force which helps return the wheels to the neutral position when the steering wheel is released.
The effects of positive caster can be seen in the motion of this furniture wheel. When it is acted on by a forward-moving force, its pivot point ahead of the wheel ensures the wheel always trails behind.
Most cars have positive caster, because it makes it easier to travel in a straight line with minimal driver action. But as positive caster increases, more and more effort is needed to turn the steering wheel.
Some vehicles have by design an amount of negative caster. Generally such vehicles would only operate at low speeds as vehicles with negative caster can become unstable as speed increases.
In all cases, the manufacturer’s specification should be followed.