Interactive Fiction has been defined many times over so rather than put it all in my own words I'll defer to those with better words and more experience in the medium. Introducing Interactive Fiction is an excellent place to start. It is part of the Brass Lantern site maintained by Stephen Granade.
For a full discussion of the heritage, theory and future of IF Nick Montfort's Twisty Little Passages - An Approach To Interactive Fiction is the place to look.
There is a thriving IF community on Usenet. Take a look here for an introduction.