Calendars are one way to measure time. They give each year a number.
The most widely used calendar is the Christian one. This counts backwards and forwards from when Jesus Christ was born. It counts forward from his birth, putting AD in front of the number. (AD stands for Anno Domini, Latin for 'the year of Our Lord'). We can leave AD out if the date is clearly AD. So we usually say the Vikings captured Walcheren in 837, not AD 837.
The letters BC stand for 'before Christ. BC dates are counted back from the birth of Christ, so they get bigger the further back in time you go.
With AD it is simple: 2003 is longer ago than 2013.
With BC it is the other way around: 2013 BC is longer ago than 2003 BC.
The Christian calendar is not the only calendar. The Muslim calendar dates backwards (BH) and forwards (AH) from the date Muhammad was driven out of Mecca. The year 622 AD in the Christian calendar is the year 1 AH in the Muslim calendar.