If your website is accessible with and without the www subdomain Google will see your site as two different sites. When your home page, http://mydomain.com/ and http://www.mydomain.com/, get indexed as two separate pages their page rank gets diluted because their content is duplicated.
For usability reasons you sill want people to be able to type in and use both domains. To get around this on Apache servers you can create a .htaccess file that redirects all traffic going to mydomain.com to www.mydomain.com. This script rewrites the domain and preserves the rest of the URL and should nearly transparent to the user. It uses a permanent 301 redirect which Google will recognize and reindex any links as www.mydomain.com.
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^mydomain.com [NC]
RewriteRule ^(.*)$ http://www.mydomain.com/$1 [L,R=301]
[...] – bookmarked by 3 members originally found by kingdomhearts449 on 2008-07-22 Avoid Page Rank Dilution With .htaccess http://www.forgeniuses.com/?p=89 – bookmarked by 4 members originally found by LisaNovaNude on [...]