Ring Flash and Macro LightsA ring light is a flash that encircles a lens and produces a 360 degree output of light onto a subject. A large ring light produces uniform light that is shadowless. When the subject is close to a backdrop or wall, it also will produce an even, all over shadow around the subject as if lightly outlining it. Ring lights are often used in fashion and beauty photography.
Another effect of using a ring light is the circular catchlights that are seen in a persons eyes. These catchlights are basically a reflection off of the eyeball of the light source that causes specular highlights. The highlights illuminate and make the eye stand out. See the small white rings in the eyes above?
Notice the even light on the subjects face above, with no harsh shadows. Notice the faint, even shadowed outline around the subject. Both of these are typical results from using a ring flash. The blue highlighted vignetting is caused from attaching a macro flash onto a wide angle zoom lens, and zooming all the way out. While this effect is not "supposed" to be captured, there are many fun things you can do with a ring light/macro flash such as this to add another dimension to a photograph. The best advice is to experiment and then experiment some more!