Before the closed-door conclave to elect Pope Francis' successor, Catholic cardinals have gathered in the Vatican for a final Mass and exhortation.

Simply put, a conclave is the assembly of cardinals that elects a pope. The word itself comes from the Latin "com" and "clavis," meaning "together" and "key," respectively – highlighting the absolute, behind-closed-doors secrecy with which the cardinals conduct their discussions and balloting.

The papal conclave begins May 7. Here's how cardinals choose the next pope, what to expect and how secretive the process is.