If backlighting is what happens when your subject is lit from behind while you expose for their face, then silhouettes are what happen when a subject is lit mostly from behind and you expose for the light.
Silhouettes are created in extreme lighting situations when the light behind your subject is bright and the light on and in front of your subject is low. When you expose for the bright background, you render your foreground and suject to be darker, giving you a silhouette. It's a great storytelling trick that puts just the form or the "idea" of your subject completely in context with it's background. Just look at this story told at the beach near sunset...
When using a point and shoot camera or shooting in automatic, turn off your flash and the camera will do the work for you - automatically exposing for the brightest part of the image (the backlight). When shooting in manual, spot meter for the background (the brightest light) by pointing your camera at the backlight and get your settings from there!
One of the best things about Silhouettes? They're easy to accomplish with just a little bit of trial and error and they go a long way to help you understand how your camera handles exposure. Artsy and informative - what's not to love?!?