Sometimes you may find that you have both www and non-www versions of your site e.g http://www.mysite.com & http://mysite.com indexed in the SERPs which not only looks messy but could possibly lead to your site being penalised for duplicate content. One method to prevent this is to use Apache's mod_rewrite to change the URLs.

This is how you would use it to change non-www to www

and this is how you would do it in reverse i.e. change www to non-www URLs

There's much more that can be done with Apache's mod_rewrite rules. To use them to their full potential, I'd recommend acquainting yourself with regular expressions for which you'll find many tutorials on the net.

__________________
Lawrence