In d00dspeak and general mockery of it, ROMz refer to ROM images.
These ROM images are (in warez sense) ripped from game cartridges, usually from old games that can be considered abandonware - usually, old NES/arcade ROM images are being distributed very publicly because the copyright holders (wisely) don't bother to stop the trading of out-of-market games.
In the "underground", people are trading more recent stuff, such as N64 ROM images.
(And the only ROM I've ever distributed to warez folks was the "PONG" N64 game... =)