This is one of the resources I often turn to for looking at some of the basic assumptions of postmodern lit:
John Lye's Explanation
Foremost, I think, is the notion that postmodern lit is self-relexive, that is, lit. that's aware of itself as a construct, a work of fiction rather than as a mirror of reality or an imitation of reality. A good example of this in short form is John Barth's story "Lost in the Funhouse" from his collection by the same name. And, believe it or not, its fun to read.
Here's a link to the story. The first two paragraphs will give you the general idea.
Lost
If you get through part of the story, you will also notice the references to Joyce's Ulysses along with Barth's parody of Joyce's style, another feature of postmodern texts.