Check out the Wikipedia entry for some detail. But essentially, the system was devised during the middle ages in Europe, when a lot of different systems for years were used. read more
And A.D. Stands for "Anno Domini" which I believe means "in the year of our Lord." At 0 A.D. is the time that Jesus Christ was born. It's just a way to separate the time before the Saviour was sent and after. read more