When an object moves in a circle, which is effectively what a car does when it rounds a bend in the road, the moving object must be pulled or pushed inward toward what's called the center of rotation. It's this force acting toward the center -- the centripetal force -- that keeps an object moving along a curved path.

Centripetal force prevents moving objects from exiting a curve and flying off in a straight line by continuously making them change their direction toward the center of rotation. Gravity plays an important role in this. On Earth, gravity applies a constant downward force on all objects. For a turning car, the downward force of gravity is matched by the upward force provided by the surface over which the tires roll. So gravity creates the conditions for the friction between the tires and the ground so that, in turn, the ground can provide the push (that is, the centripetal force) needed for the car to turn.

Often, people confuse centripetal force with centrifugal force. The sensation passengers (or even fighter pilots) experience that makes them feel like they're actually being pushed away from the center of rotation -- or "thrown" -- during a sharp turn is commonly referred to as centrifugal force, although it isn't a force at all. It's the result of observing one's motion relative to the object in which one is traveling.

To better understand the distinction, put yourself in a passenger's situation. When a car you're riding in suddenly changes direction (for example, takes a sharp turn to the left), your body continues to travel in the same direction it was traveling in before the abrupt shift. As with the rest of the car, the passenger door starts to move in a circle due to centripetal force. To continue your motion in a straight line, you would have to go through the door. As a result, you will find yourself pressed against the passenger door. If you were to observe your motion relative to the car, however, you'd realize that you're actually being pushed by the door inward to the center of rotation, thus prevented from moving in the same direction you had been traveling.