"That's the beauty of erotica, it can add to any existing genre, or stand on its own." So says Michael Bracken in Writing Erotic Mysteries
"Erotica is not pornography, though it can contain pornography. The primary difference is that the single purpose of pornography is to sexually arouse the reader, period. There is not much of a plotline, if any, and there is no character development. Erotica, on the other hand, tends to have a genuine story, which helps to emphasize the erotic elements. There may be character development through the course of the story, and there is a much greater emphasis on the thoughts and emotions of the characters. Make sure you know which of these you are writing before you start." Written by Poison Pen & posted, along with other basic tips for erotica writers, here.
For a very basic overview, see Erotic literature