• BC means *Before Christ*. As I write this, the year is 2012, AD (which means, by the way, *Anno Domini* - the Year of Our Lord). Jesus Christ's birth is the basis of the calendar that we in the Western World have been using for over 2000 years. Actually, Jesus was born four years earlier, which historians found out much too late to change anything. But for all practical purposes, Jesus was born in the year 1. Anything that happened before his birth is BC.
• BCE means *Before the Common Era*. For people who would like to use the same time frame as BC, but don't want the *Christ* connection. The timeframe is exactly the same. So BC=BCE
• BP means *Before Present*. This works OK for very ancient timeframes, but not so good for recent history, as you would have to always go back and revise it every year.
I hope that this helps.