Ring flash units work almost exactly as normal external flash units, but are specifically designed for macro photography.
Since your lens may be very close to the subject when shooting macro, a normal flash unit will not help you enlightening your small subect. For macro photography, you need a softer light which can enlight very closed subjects.
There are two types of macro ring flash units:
There are pros and cons for both the solutions. The first type of units make a softer light from all around the lens, which produces a smoother and clean result, but it is not adjustable. The second type of units make a harder light on the subject if used without diffusors, it only enlight from where the units are positionated, but it can be regulated based on where the subject is and this is absolutely the best feature for this kind of flash unit.
Both the solutions have the possibility to activate only a part of the flash, for example the right side or the left side to create nice light effects on the subject.