The following websites provide explanations of literary theory generally as well as overviews of different theoretical approaches:
Using Key Passages to Understand Literature, Theory and Criticism by 
Literary Theory: A Guide for the Perplexed
Literary Theory From Plato To Barthes: An Introductory History
Literary Theory and the Claims of History: Postmodernism, Objectivity, Multicultural Politics
The Oxford Dictionary of Literary Terms
The Concise Oxford Companion to English Literature
