Defining EroticI think many people have tried to define it, but for me, the most interesting part of something erotic, as opposed to purely pornographic, is the issue of "participation" -- that is, an erotic photo or story like this, leaves much to YOUR imagination.
The erotic demands your participtation, allowing you to take something something slightly vague or blurry, not explicit, and make it "yours" by adding the specifics in your mind.
Like a good novel, you supply a lot of the imagination -- what the characters sound like and look like, the furniture in the rooms they inhabit. It's a medium that requires a joint effort on the part of the reader and the writer.
In pornography, nothing is left to the imagination and is therefore instantly boring and dull.