A caster wheel is a type of wheel that pivots. Caster wheels help when a robot is turning because the wheel pivots to roll in the same direction that the robot.

 

Here is an example of a tricycle robot. It doesn't turn very well because there is too much friction between the fixed front wheel and the ground.

If the front wheel is replaced with a caster wheel, then the robot turns much more easily. The rolling wheel offers much less friction than the fixed wheel.
Here are some other designs of caster wheels.  
 
See also: Friction