Caster describes the angle c between the kingpin and the vertical plane. In case of a double wishbone-type of suspension, the axis through the centers of the ball links serves as a 'virtual hinge pin'. If the kingpin is leaning back, as in the pic, the caster angle is said to be positive. Negative caster (kingpin leaning towards the front) is never used. Note that the contact patch between the tire and the ground is behind the intersection point of the extension of the kingpin and the earth. (Dimension d) This will cause the wheels to 'trail'.
The caster angle will cause excessive camber in the front wheels as they are steered, lifting the front of the car up. This lifting effect is what causes the front wheels to have a tendency to straighten out when there's no steering force applied: when the wheels are pointed straight ahead, the chassis sits at its lowest position, steering the wheels requires some force, to lift the car up. When the force is removed, gravity will return the wheels to their original position. The bigger the caster angle, and the heavier the car, the stronger this effect is. Also, the bigger he caster angle, the bigger the camber difference induced when the wheels are steered. This camber difference is to compensate for the chassis roll and tire squirm when the car is cornering. Hence, a lot of caster will provide more steering in high-speed corners, where chassis roll is more pronounced, and whilst turning in. It will also make the car more stable in rough conditions, and the car's straight-line stabili ty will also be improved. A small caster angle will provide more steering in low-speed corners, and less turn-in.


Note that the caster angle isn't always constant. Cars with double wishbone suspension, where both wishbones aren't parallel, will experience a change in caster angle as the suspension is compressed or extended. If the lower wishbone has less kickup than the upper wishbone, the caster angle will decrease as the suspension compresses, like when the car is turning or braking. This is called 'reactive caster'.