Images is a great site for film and popular culture. They have a number of articles on Westerns, but I suggest you start with "The Western, An Overview." This seven page article covers the history of Westerns in film. Here are a few Images links.
The Western, An Overview
The Silent Western as Myth Maker
If these articles have whet your appetite for Westerns, I have a suggestion:
The Steve Dancy Tales
Honest Westerns ... filled with dishonest characters.
and coming soon...