It's impossible to determine an exact limit between the erotic and the pornographic. I have often, among friends, discussed it over, trying to debate the difference between the two, but the conclusions have always been controversial. Some admit that the pornographic is characterised by the explicit exhibition of sexual acts, as well as parts of the body, including the genitals, whereas the erotic deals more subtly with the presentation of the bodies.
However, from my point of view, the barrier isn't that obvious. You could distinguish eroticism from pornography through the participation, or lack of it, of the viewer in the sexual show. In pornography, there is a participation, through sexual arousal, while in eroticism, if if there is the awakening of desire, it isn't the main goal of the work to wake it up, but rather demand a distant intimacy, basicly seeking the knowledge of desire and pleasure that, at its limit, is a way of pleasure.
To represent this controversy, I leave you with Vlastimil Kula. Art or Pornography?