B.C. means "before Christ," and A.D. means "anno domini," or "year of our Lord." Therefore, the timeline largely in use as of 2015 is based on what is believed to be the birth of Jesus Christ. Non-Christians may use B.C.E., or "before common era," or C.E. for "common era."
This system does not contain the year zero. It was developed in 525 A.D., which, because of this feature, is considered the sixth century. It was not used extensively until after 800 A.D. Dionysius Exiguus, an eastern European or Russian monk, created the system. He wished to replace the old Diocletian 247 system, because the Emperor Diocletian promoted the persecution of Christians.
Dionysius based the system on his supposition that Jesus was born 753 years after the founding of Rome, which turned out to be incorrect. However, scholars do not know how Dionysius calculated this date.
When the B.C. and A.D. systems became widespread, people in Christian countries used dates from the life of Christ to signal the beginning of the new year. For some people, the year began on March 25, which was supposed to be the date when Jesus was incarnated. Others used December 25, the date of Jesus' nativity, as the start of the new year. A few people used Easter, the day of Jesus' resurrection.