Rom cards have patches, combies etc. already programmed and are not able to be edited. Basically like adding a new bank to your synth. Ram allows you to edit and save your own sounds, rewrite over them, etc.
PCM cards have new samples (voices) and are not rewritable. These cards are not universal...a Korg Wavestation EX won't work with a Wavestation SR or Rolands, Alesis Kurzweill's...