A caster wheel is a wheel equipped with counterbalance steering, which allows it to rotate itself in the direction it is being propelled.
Caster wheels exist in two basic types: rigid or swivel. Rigid wheels are mounted and fixed between forks, which allow them to only move in a straight line. Swivel wheels can rotate 360 degrees, allowing them to move in any direction.
The units these wheels are mounted to can be equipped with any combination of the two types of casters. Generally the set up is two rigid and two swivel caster wheels, allowing the unit to go straight or pivot at turns; or four swivel casters, which allow the unit to move in any direction required.