This is tutorial covers building a macro ring-light and is intended for people who own a consumer type of digital camera with macro capabilities. You don't even have to take your camera apart to accomplish this. bonus!
Check out this amazing cost saving how-to build your own Macro Ring-light.
A ring-light can be useful when photographing from really close by (macro). When holding your camera close to the subject you are likely to block the light coming from another light source. Using the flash won't do much good either. It's positioned above the lens, the lens barrel will probably block it, otherwise it would overexpose the scene because it will be too bright.
There are some macro flashes around, but they're very expensive and intended for SLR camera's not the consumer typed digital camera. Because the light comes from different angles the light is equally divided and the scene is perfectly lit.