When you look closely under a mirror and you see little holes on your skin, those are open pores. Some people have very large pores (usually oily skin) and some have very small or invisible pores. The oilier the skin, the more noticable they are.
They pop up on the cheeks, forehead, and chin mostly. Blackheads and whiteheads are the results of clogged pores.
Go to:
womenfitness.net/beauty/skin/skinproblem.htm
It is a great place to get info on all kinds of pore problem and how to spot a pore.