Key light:

This is typically the strongest light. It is also the primary source for shadows.

Fill light:

This light is softer than the key light. This light is meant to compliment the key light. Notice how in the image above, even though the key light is quite strong, there are still areas that the key light does not reach. The fill light is placed to reach those dark spots that the key light does not reach.

Kicker light:

Or sometimes known as the backlight. The job of the back light is to bring out the silhouette of the objects. The back light helps to separate the subject from the background; this directs the viewer's eye towards the subject.

The back light should be placed behind and above your subject. The light can be as bright as necessary to draw out the subject from the background.

All three lights:

Light is a crucial part of any 3D scene. Good lighting can add to the mood and realism of a scene, while bad lighting can detract from the clarity of the image or animation. Three-point lighting is a technique used to light subjects in traditional photography and film. The principles of 3-point lighting apply very well to 3D animation.