I found some good web articles to explain in more detail what a print.css is and what it does.

http://www.intensivstation.ch/en/css/print/

http://www.alistapart.com/articles/goingtoprint/

http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9005339

http://envisionic.com/webtips/user_experience/printer_friendly.php