1. White Heads: These result when large amounts of puss build up under the surface of the skin. These are the most common type, and are the most satisfying to pop, as long as you're careful where you're aiming. These are easy to remedy, but most solutions are only temporary.
2. Black Heads: Deeply inground dirt can also clog the skin; however, unlike surface blemishes, these fester more deeply than their lighter colored cousins. They also may produce puss in addition. They are much more painful (and therefore much less fun) to pop. But, once popped, this variety will not return.
3. Ingrown Hairs: These are caused by scaredy-cat ostrich type hairs that don't like the surface, and bury their heads in the ... skin. When mashed, as other types, the hair no longer has a place to bury its head, and has to face the world ... head on.
4. Scar tissue: These are former, washed-up, "used-to-be" puss-geysers that have been dealt with improperly too many times before. THESE HAUNTINGLY FAMILIAR HAS-BEENS WILL NEVER GO AWAY!