The Common Era refers to the conventional Gregorian numbering of years from an epoch based on the traditionally reckoned year of the birth of Jesus Christ. This convention for year numbering was introduced by Christians in the 6th century, although it was not widely used until later. Years before the epoch were denoted A.C.N. (for Ante Christi Natus, Latin for "before the birth of Christ"), although B.C. ("Before Christ") is now usually used in English. These abbreviations are placed after the year number, which is counted backward from 1; that is, the first year before the epoch is "1 B.C.", the second year before the epoch is "2 B.C.", etc. Years after the epoch are denoted A.D. (for Anno Domini, Latin for "in the year of the Lord"), with the abbreviation before the year and counted from 1. So the first year of the epoch is "A.D. 1" (there being no year 0), etc.
C.E., an abbreviation for "Common Era" or "Christian Era", is equivalent to A.D., but is placed after the year, and is preferred by some in secular writing. B.C.E., an abbreviation for "Before Common Era" or "Before Christian Era", is equivalent to B.C., and is likewise placed after the year number.
Due to the dominant influence of Christianity in the development of Western civilization over the last 2,000 years, the initials B.C. (or A.C.N) and A.D. have been used without question for many centuries. Rejecting the historical domination of Christianity over Western civilization, the movement to substitute B.C.E./C.E. for B.C./A.D. has grown along with political correctness in Western academia.
See also Anno Domini.