In simple terms, I would say porn is tasteless, whereas erotica is tasteful. Porn is lustful and/or dirty (and also sometimes dirty in other meanings of the word), but erotica is more sensual. Porn is professional, cheap (in the sense of shoddy), cheap or expensive (money) and nasty (crap), and nasty in other ways; erotica is amateur, still professional but not 'empty'. Porn is fucking; erotica is lovemaking. Porn invokes lust and power/control; erotica invokes more than just lust/not lust at all. Porn is a quick get-off; erotica is a love-making sex scene.
Maybe it's none of these things. Maybe porn is for the commoner, where erotica is for the "higher" class/es.
Of course, things overlap and lines become blurred, but I would think that generally would be a somewhat accurate way to differentiate between them. Maybe that's just my opinion, though. If you search for the difference between them online, you'll find enough sites trying to guess.