The quick and cynical answer is the difference involves money and class markers.
Erotica is...expensive porn. Erotica is porn that's expensively produced and marketed. Or porn that's set among the rich and beautiful. Or porn that's accessed in ways that emphasise class markers--- expensive editions, expensive techniques of production. But there's something else, too: the idea of abstraction and self-consciousness.
When I was fourteen or fifteen, I acquired a copy of Pauline Reage's "Story of O". Yes, that was the white-covered Grove Press edition, the one that's still in print, the one that was sold in hip bookstores in university neighbourhoods. I'd heard of the book. It was a title I'd seen mentioned in discussions of Grove Press, which was itself a class marker. Grove in those days was the American press that specialised in translations of the Parisian literary underground. They did editions of Beckett, they did editions of surrealist works, they backed the Evergreen Review, which was the hip art review of the late 1950s and early 1960s. That's a whole set of class markers on its own. I knew that "Story of O" was "an erotic classic" and that it was French. I took a copy home from a tiny bookstore filled with exactly what you might expect--- Beat novelists and poets, French and South American existentialists, utopian Marxist and anarchist books. I read it and was just...stunned and amazed.
"Story of O" is of course the s/m classic. Sitting in my room that first day, I had no idea what it would actually be about; I'm not sure that I quite knew what s/m was. Of course the book was a major sexual awakening for me. It opened up avenues of sexuality that I hadn't quite imagined. I knew porn existed, of course. Now this was long before the internet, and long before video. There were porn novels, of course. Pulp tales in cheap editions, usually with solid-colour covers. I knew those were on spinner racks in bus stations and convenience stores in bad parts of town. I knew those things existed, even if I hadn't read them...or read more than one or two (and we won't discuss the possibility of shoplifting here). "Story of O" was...amazing and different.
The s/m scenes in "O." are classics of their kind, of course. And it's almost a cliche--- the number of girls who claim that reading "O." in high school changed their lives. "Corrupted" is the word the Victorians would've used. But there's more to it than the sex, hot as those scenes are. For me, "Story of O." opened up a second set of fantasies, something past the sex. You know the idea. O. is taken into a secret world of hidden chateaux and private parties and elegant townhouses and converted abbeys in the old-money parts of Paris and environs. And O. herself is a fashion photographer by trade. Even before she's taken into a world of wealthy men who own her, she moves among the beautiful and stylish. "O." wasn't just about girls being whipped and chained and violated, it was about secret worlds. The novel deployed class markers just as much as it deployed images of subjugation. "O." was aspirational--- it was about a whole parallel, moneyed world with intricate rituals.
There's the literary aspect, too. And not just the style and the grace of the writing, though those things are class markers, too. "O." made clear reference to things like "Letters From A Portuguese Nun", to French high lit. The story came with a pedigree, and one its readers were expected to know. It came with theory built into it, and that was part of its power.
Porn is instrumental. It's written to get the reader off. Erotica is designed to make the reader think about the idea of getting off. There's a level of abstraction there that bus-station porn didn't have and didn't bother with. There's a distinction there worth exploring, and one that was something to do with the definition of art. Porn is about producing an immediate effect. Erotica, like self-conscious art, is about making you think about the idea of an effect. Erotica wants the reader to be aware of what's being done to create an effect. There's something of that in high-art painting, where the viewer is supposed to be aware that what he's seeing is a painting, that the painting has a history and that the artist is aware that the viewer is aware of what's being done to create an effect. There's a self-consciousness in erotica that isn't there in porn. And there's a presumption that the reader has the time to consider that self-consciousness, that the reader has time to apply knowledge about lit and art to the work.
So many of the usual list of erotic classics are about s/m, and s/m is always the intellectuals' fetish, isn't it? It's about abstractions, about theory and scenarios. Even the costumes and equipment are designed to abstract participants from the mere physical sex and into ideas. And s/m is an expensive sexual taste when done the way it's done in lit. That's no accident, I think. Which makes me think of another class marker, too. Erotica, whether s/m or not (though s/m offers a clear example), also has time built in to its stories. Beautiful locations, beautiful costumes, elaborate scenarios and games--- those things don't just take money to access, they require time. Time to prepare, time to play out. And here in the new century, having time is even a more of a class marker than money.
So...porn v. erotica. Do you have an instinctive grasp of which is which? Do you know it when you see it? I'd be interested in your thoughts and views. I've talked a lot about s/m here, but I think some of this applies to other things as well. David Hamilton's photos from the early and mid-1970s are beautiful and dream-like and breathtakingly erotic. But it's not just that they're lovely young girls naked with one another in the Provencal countryside, and it's not just that soft-focus is some kind of criterion for saying "not porn". It's that Hamilton's photos are supposed to make the viewer instantly think of their pedigree, of paintings and poetry that deal with the same kinds of image...and that Hamilton expects the viewer to know about, say, Pierre Louys. Erotica is about sex, but it's less immediate than porn. It adds that extra step of self-consciousness.
There are other images that occur to me. High-fashion erotica, of course--- e.g., Helmut Newton or the work Jeff Dunas and Rebecca Blake did in the 1970s. Again, there's the idea of self-consciousness. Money, too, yes: expensive hotels, distant cities, elegant houses. But always that intermediate step between the image and physical arousal, that small disconnect.
Tell me what you think. What defines each for you?