Koichan's answer is correct, with one tiny exception: anno domini means in the year of THE Lord; anno domini nostri means in the year of OUR Lord. We don't know the date of Jesus' birth. The Catholic Church chose December 25th, because the daylight hours have started lengthening by then: Jesus Christ is the Light of the World. I hope this helps answer your your question fully. Additional questions? Please leave a comment. Frank
The Politically Correct version of that - so as not to offend the sensitivities of religious folk of non-Christian faiths - is to accept the Year 0 as being the start of the Current or Common Era. So you will find years are called BCE (Before Common Era) and CE (Current Era). That's official, but I don't know whether to laugh or cry.... Welcome to the 21st century and 'correct speak.'
It is commonly thought that BC stands for "before Christ" and AD stands for "after death." This is only half correct. How could the year 1 B.C. have been "before Christ" and 1 A.D. been "after death"? BC does stand for "before Christ." AD actually stands for the Latin phrase "anno domini" which means "in the year of our Lord." The B.C. / A.D. dating system is not taught in the Bible. It actually was not fully implemented and accepted until several centuries after Jesus' death. From History for Kids