A.D. is the abbreviation for the Latin "Anno Domini" which means "Year of our Lord". In terms of historical dating conventions, it means, "The Christian Era", starting from the year A.D. 1 as the supposed year of Christ's birth.
A.D. is gradually being replaced by the abbreviation "C.E." which means "Common Era" and is identical to the notation and dating of "A.D.", only the name has changed. C.E. is also known as the "Christian Era" or "Current Era."
B.C. is the abbreviation meaning "Before Christ". It is now being replaced by "B.C.E." meaning "Before the Common Era" and is also taken to mean "Before the Current Era" or "Before the Christian Era."
I hope you have found this helpful.