Key Light - The key light is the main light. It is usually the strongest and has the most influence on the scene. It is placed one side of the camera/ subject so that the side is well lit and the other side has shadow.
Fill Light - This is the secondary light and is placed on the opposite side of the key light. It is used to fill the shadows created by the key. The fill will be softer and brighter than the key. To create this, you could move the light further away.
Back Light - The back light is placed behind the subject and lights at either side of the back light. The purpose is to provide and subtle highlights around the models outlines.