The Caster of an automobile refers to the angle of inclination of the front wheels towards the front of the car. It is the adjustment which causes a car to drive in a straight manner rather than wandering. Toe-in can also affect this, but only because it makes a car more stable.
Most vehicles do not have adjustable caster, but it is possible to adjust the caster nonetheless via a variety of methods. The simplest, used on cars with a MacPherson Strut Suspension up front, is to simply adjust the positioning of the upper strut mount. Most vehicles do not have an adjustable strut mount, but you can get them from a variety of automotive aftermarket performance shops. They are available to adjust camber, caster, or both. On some cars, you can also adjust the steering knuckle pivot points. Finally, on some vehicles it is necessary to adjust camber by shiming some part of the suspension, or by the use of eccentric bushings in the end links of the suspension members.
Positive caster, as depicted in the diagram below, means that the steering axis (the diagonal line in the diagram) points towards the front of the car as it descends. The more caster you have (That is to say, the more positive your caster is) the more the car will tend to center itself, and the less it will wander. However, increasing caster increases the amount of force needed to steer.
Most cars have +2 to +6 degrees caster as they come from the factory. Each side of the car should have caster as close to the other as possible, and at worst within 1/2 degree.
Diagram showing caster angle:
\ Caster Angle
|
\
| Vertical Line
\
| Front ------>
\
|
\ _,. _,-. <--- Spring Perch
.-'_,-' _,` _____|_____
\.-\ _,-'__,-''" ``--..
,-' _,-' \-' | `-..
/ ,-' _\ \ \ `\.
`' ./ \ \ | `.
,' \ \ \ `.
_/ \ \ _,-, | `.
/ \ \ \,-' / Wheel \
/ _\,-' _,\' | `.
' _,-' _,-\' \ \
/ '._,-\ \ | `.
/ \ \ \ \
.' \ \ | \
| \ \ \ '.
.' Shock ---> \ | \
| \ \ \ |
| \ | \ |
| \ \ \ |
`| \ | \ .'
\ \ \ \ /'
`. \ \ /
\ \ _,-' .'
\ \,-' .'
`. .'
`. .'
\ ./
`. .'
\ .'
`-_ _'
`-_ _-'
`-_ _'`
`--.____________________,-''
This diagram omits the spring, since it is excessively difficult to accurately represent in ASCII.