B.C. means 'Before Christ' and is used to show the number of years before Christ was born.
For example Stone Age People lived in Ireland in 9,000BC.
This means 9,000 [nine thousand] years before Christ was born.
A.D. means 'Anno Domini' [After Christ]. It is a latin phrase which means 'In the Year of the Lord'.
When AD is used beside a date it means that it is a year after Christ was born.
For example AD2007 is 2,007 [two thousand and seven] years after the birth of Christ.