Before and after the death of Christ.
Starts with year one. We're in the year 2011 AD, or CE. Everything counts backwards from before that.
AD and BC are both ways of describing historical time. AD stands for "Anno Domini" which is Latin for "year of our Lord," It refers to years 1-present. BC stands for "Before Christ" and refers to dates before the year zero (the BC time line runs backwards, so 575 BC is older than 5 BC).
BC= before christ, AD= after death of christ
People who want to use the most widespread (Gregorian) calendar but without religious reference use BCE and CE instead of BC and AD. BCE=Before the Common Era, and CE=Common Era.
Mostly correct, However, there was no year zero. If we knew the exact year Jesus was born (which we don't) the year before AD1 would be 1 BC.