Also a catch phrase from the television show Monty Python's Flying Circus. (Contrary to the implication in the writeup above, the name of the comedy troupe responsible for said show was "Monty Python", however this only came about after the name of the show)
Although there was no universal pattern to the shows, and the phrase was certainly used in other contexts, the most common instance would be an opening sketch, at the end of which the camera would cut or pan over to John Cleese sitting at a desk (usually in a rather unusual location). He would speak the line, the camera would cut to Michael Palin, dressed as a castaway (Thanks, AndieX), who would say "It's...", and the opening credits would roll.