Hi! What's the difference between "domain.com" PR and "www.domain.com" PR ? If these two PR's are different what does this mean ? Thanks.
PR is by page, not domain. It happens that Google and the other SEs see www.example.com as something different than example.com. You should either a) point all your links to one consistent version, or b) 301 redirect one to the other.
This is not an unusual occurence. Picture the PR value similar to a quality measurement. Much of that value is based on "recommendations" from others (e.g. other sites. If the links, both within your site and from outside pointing to your site, use one or the other more predominately you will achieve the results you are questioning. The real answer to your underlying question is to pick one or the other and promote it. Drive the traffic, the PR value, etc. to the one destination. That is not to say promote only one page. No way! Promote all the pages you can if they have valuable content that people will want to reach.
Assuming you are on an Apache server. Options +FollowSymLinks RewriteEngine on RewriteCond %{HTTP_HOST} ^yourdomain\.com RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [R=permanent,L] in your .htaccess file will do the trick.