Possibly you mean 'BC' and 'AD'. BC indeed means Before Christ, i.e. the years counting back from the supposed year of Christ's birth. AD means Anno Domini, a Latin expression translated as 'in the year of our Lord' i.e. counting forward from the supposed year of Christ's birth. Our present year is 2008 AD, so Christ's birth year is supposed to have been 2008 years ago, but nobody knows precisely.