Porno, pr0n, peep shows...You get the idea.
An etymological note: An "X" rating used to mean a film wasn't rated. (In fact, one X-rated movie, Midnight Cowboy, won an Oscar.) Pornographers got hold of the X in the 70s, and soon the X became associated with ill-repute. XXX is just a way of advertising it.
I'm probably the last user on the planet who hasn't seen Pam and Tommy's XXX video.