What is centripetal force?

When an object is spun in a circle, it is constantly accelerating, even if the speed is unchanging. This is because accleration is a change in velocity and velocity is speed and direction. A spun object is constantly changing directions. By Newton’s first law, objects tend to keep moving in a straight line unless acted on by a force, so a force is required to change its direction and keep it going in a circle instead of a straight line. For instance, if you wave an object on a string in a circle, the tension force of the string is the centripetal force.

http://xkcd.com/123/