Intense pulsed light is a high technology way of removing hair.  A very bright flash of light is used and directed at the skin.  The light energy is picked up by the hair which is then heated up.  The hair in the follicle therefore burns the cells in the follicle wall, hopefully destroying them.

Of course the flash of light also hits the skin.  If the skin is dark, then the skin can also be damaged in the same way.  If the skin is light, there is less chance of skin damage but it is still possible.  Therefore intense pulsed light machines produce not just one flash, but a series of flashes (usually 3, 5 or 7) with a very short rest in between each.  This is done so that the dark hair absorbs the energy from the flashes, allowing the heat to build up while the lighter skin cools between each flash. In this way, more damage is done to the hair follicles whilst little damage is done to the skin surrounding them.

From this explanation, it can easily be understood why IPL works best in people with dark hair and very white skin.  The darker the skin, the more the heat is absorbed by the skin.  The lighter the hair, whether it be light blonde or grey, the less the hair absorbs the energy and the less IPL will work.

The advantage of IPL is that it is relatively quick and high-tech.  It does not cause permanent hair removal but does cause permanent hair reduction.  Hairs can grow back but they are usually thinner and lighter.  The disadvantages are that it is relatively expensive and can be quite tender, particularly in sensitive places.