Hey, maybe it'll help if you know what they stand for;
A.D. = Anno Domini, Latin for "In the year of our Lord". Everything after Jesus Christ is A.D., for example this year is 2009 A.D. (Think of Christ's time as Year Zero)
B.C = Before Christ, speaks for itself really. This counts backwards because if it's 900 B.C, then it's 900 years before Christ, so 100 years later it would be 800 B.C, which would be 800 years before Christ.
B.C.E is the same as "B.C." but stands for "Before the Common Era" - preferred by non-religious people.
C.E is the same as "A.D." but stands for "Common Era", also preferred by non-religious people because it doesn't refer directly to Christ.
So in short - B.C or B.C.E. is before Christ/before the Common Era, and counts backwards.
and therefore A.D or C.E. is after Christ/the Commone Era, and counts forwards.