AD indicates Anno Domini and translated this means
In the year of The Lord.

A date written as 183BC means 183 years before Jesus was born.
A date written as AD 43 means 43 years after Jesus was born.

I write this in the year AD 2011.

Note: People who are afraid of offending non-Christians or who deny Jesus will insist on using the letters CE (Common Era) to indicate the number of years after Jesus was born and the letters BCE (Before Common Era) to indicate before Jesus was born.
Counting up to and after the birth of Jesus is the reason we have the system but some people seem to struggle with that truth.