Common Era (C.E.) is used instead of A.D., because A.D. means "the Year of our Lord," and some people do not believe that Christ is our Lord. Many people use B.C.E. and C.E. instead of B.C. and A.D.

B.C.- Before Christ
A.D.- Anno Domini (Latin for "the Year of our Lord")
B.C.E.- Before Common Era
C.E.- (See Above)