The Standard 3-Point Lighting Technique
One of the oldest lighting techniques in the book is called "Three Point Lighting". It is vastly used in studio photography and by snobby fashion photographers. It is also a very good basis for any portrait photograph. In this technique you use three lights:
The first light is a key light. Usually this is the strongest light and this light sets the lighting of the scene.
The second light is called a fill light, this light helps fill the shadows that the main light casts.
The last light is called a backlight (because it comes from the back), and is used to create a contour and separation. It is common to use a snoot or a gridspot on the backlight to avoid a spill.
The guys at mediacollege have created a nice illustration and explanation of that basic technique. They have also created a cool flash simulator (After writing this, I've noticed that this is a pan, so no credits here. Kudos for the great pan) that can help understand the concept of a three point lighting. Or you can just click the various lights and enjoy seeing how the model reacts to each type of light. The flash simulator is also good way to understand key light and backlight in general.
While in general the guys (or girls, I don't know who works there) deal with video, the lighting stuff is great for still photographers as well.