B.C., which stands for “Before Christ,” is used to date events before the birth of Jesus. A.D. is the abbreviation for the Latin phrase anno Domini, which means “in the year of our Lord,” and is used for dates after Jesus’s birth. This system of dating has been used for many years by Western archaeologists.
Today, however, with a growing understanding that not all archaeologists are Christians, some archaeologists prefer to use the terms: Before the Common Era (B.C.E.) and the Common Era (C.E.), which are exactly the same as B.C. and A.D. but have nothing to do with Christianity.
The correct (common) way of writing:
Traditionally “A.D.” was placed before the year number and “B.C.” after, but many people now prefer to put both abbreviations after the numbers. (ex. 100 A.D or 200 B.C)
Additional Timbit of information:
An important piece of information worthy of sharing is that althought it may be assumed that Since A.D refers to the “year of our lord” (referring to Jesus), it is not true that Jesus was born on 1 A.D. In fact, source say that Jesus was born in between 6 B.C – 4 B.C.