When we travel in a circle, there seems to be a force pushing us away from the center of the circle.  This is referred to as Centrifugal force, from the Latin meaning "to flee from the center," and is a fictional, or relative force.  In reality, centrifugal force depends on our frame of reference - imagine that you are riding around a track in a car (your frame of reference would be the car.)  As the car turns, the centripetal force from the wheels seems to repel us outward, away from the curve (centrifugal force).  In truth, our inertia is trying to keep us moving in a straight line, without changing our acceleration, making us feel like we are being pushed away.

The following applet demonstrating centripetal force can also be used to estimate the effects of centrifugal force.  When you click on the black ball and move it, the equilibrium of the system remains intact, but the velocity of the red ball increases.  If the red ball were a car full of passengers, what would they feel as the velocity changed, and why?