B.C. and A.D. are two abbreviations you usually see following years: 1,000 B.C., 1620 A.D., 2010 A.D. Sometimes A.D. will come before the year: A.D. 400.
B.C. stands for "Before Christ". Jesus Christ is the central figure of Christian religions. A.D. stands for the Latin term, "anno Domini" which means "in the year of the Lord". This term also refers to Jesus Christ. 1 A.D. was thought to be the year that he was born. It was such an important and well-known event for Christians that it made sense as a starting point for a new calendar. Any year before 1 A.D. is considered B.C.