"One of the best forgotten pieces of net history"
ASCII porn is a type of ASCII art that depicts a pornographic image. For those of you who don't know what ASCII art is and don't want to click the link, ASCII art is a picture that is drawn from smaller symbols in the ASCII character set. ASCII art has some examples.
This ancient form of pornography was never intended to be serious. Generally it was considered a joke among the geeks of the BBS days (Late 70s until the late 80s). If you can still remember back that far you'd remember that there wasn't any such thing as a JPEG, all graphics was done in the text mode! Because of this, graphics were often hand drawn and hours of work could go into a simple image. ASCII images, and the porn for that matter, was actually considered an art by BBS goers. I doubt that anyone actually considered it to be real porn.
We forget those days all too well, but ASCII porn seems to be sparking up. Although it may not be exactly the same, ASCII porn is growing in popularity among cell phone users. This is mainly through SMS where people use the limited symbols to draw their undressed women. I imagine that as mobile phone technology progress so too will the graphics capabilities available to these 'artists'. It's certainly ironic that the mainstream users of mobile phones would probably see a geek as 'freaky' if they look up ASCII porn when it is becoming very popular among themselves.