AD stands for Anno Domini. Historians use this as reference from Jesus' birth. The use BC or Before Christ for years before Jesus. Ideally, historians wanted 0 BC for Jesus birth year, and AD 33 for His death. But they made an error of 6 years. So now, 6 BC is the official year of Jesus birth, and AD 27 as Jesus' death. Hence, the 2nd Millenium should have been celebrated in 1994, instead of 2000.
A. D. is the Latin abbreviation for Anno Domini. A.D. is used with dates in the current era, or the era since the birth of Christ. B. C. means "Before Christ". This dating system started in 525, but was not widely used until after the year 800. For more information see http://en.wikipedia.org/wiki/Anno_domini