Abbreviated as "CE", it is a euphemistic replacement for Anno Domini (Latin: In the year of our Lord), or "AD". It is used to avoid having to reference Jesus Christ in the dating system we all use.
It's basically the same idea as saying "Holiday tree" instead of "Christmas tree". In otherwords, it's fucking pointless and lame.
John: It is now the year 2010 CE, or of the "Common Era". I guess this means we're just "common" people in a "common" era.
Peter: STFU, jackass, it's 2010 AD for fuck's sake.
John: Oh, that's right, shit.