SBBE:
When looking for adjectives to use in my own writing I ran across a website called darkerotica - http://darkerotica.blogspot.com/
According to them the difference between erotica and romance is that sex has to drive the plot in erotica and not just be thrown in gratuitously, if the sex isn't necessary to tell the story, then it isn't erotica.
I found this very interesting and useful for myself.
As for the difference between porn and erotica, I agree with Ruby and think that is a more personal opinion.