Three Sources

http://plato.stanford.edu/entries/postmodernism/

This article emphasizes postmodernism's history in the world of literature and touches on its few characteristics that make it unique.

http://www.fofweb.com/Lit/default.asp

This source gives a very basic, easy to understand view of postmodernism.

http://www.cla.purdue.edu/english/theory/postmodernism/modules/introduction.html

The author of this article divides postmodernism into a number of categories, each accompanied by an example and a definition.