The earliest documented use of this method of reckoning the date is in the work of Bede in the seventh century, but the system originated with an eastern monk named Dionysius Exiguus in the year 525. The abbreviation comes properly before the date, because the phrase it stands for also comes before the date (e.g., "in the Year of Our Lord 735 Bede passed from this earth"). However, you will often see it following the date in more recent references.
A.D. and its counterpart, B.C. (which stands for "Before Christ"), constitute the modern dating system used by much of the world, nearly all of the west, and Christians everywhere. It is, however, somewhat inaccurate; Jesus was probably not born in the year 1.
An alternate method of notation has recently been developed: C.E. instead of A.D. and B.C.E. instead of B.C, wherein C.E. stands for "Common Era." The only difference is the initials; the numbers remain the same.
Some scholars still consider the Middle Ages to have begun in 476 A.D.