B.C. stands for Before Christ, and it means the number of years before the birth of Jesus Christ. That was about 2000 years ago, so the date 552 B.C. means 2552 years ago.
Some people use B.C.E. instead. That stands for Before the Common Era, and is used in order to avoid Christian references. Quatr.us feels that since this is a Christian dating system, based on when Jesus was born, it’s misleading people to try to hide that.
Why don’t we just say 2552 years ago, if that’s what we mean? Why would that be inconvenient? (scroll down for the answer).
*The answer is, because we keep going forward in time. So if you wrote in a book that something happened 2552 years ago (or whatever), the next year the books would still say 2552, but now it would be 2553, and the year after that it would be 2554 years ago. It would be too confusing.