Until recently, most history books written in English used the terms B.C. and A.D. to place events in time.
A.D. stands for the Latin phrase Anno Domini, “the year of the Lord.”
B.C. stands for the English phrase “Before Christ.”
The term A.D. was thought up by a Christian monk. He calculated Year One of the Christian calendar from the year in which Jesus was born. This method of dating was a huge improvement over the multitude of methods that historians were using. Nowadays, however, the notations A.D. and B.C. are being replaced by C.E. and B.C.E.
C.E. stands for “Common Era.” Year One of the Common Era is the Year One previously thought to have been the birth year of Jesus. Modern scholarship now dates the birth of Jesus to 4 B.C.E., the last year of the reign of Herod the Great.
B.C.E. stands for “Before Common Era.”
For more on historical dating, see Writing About History