Short definition to what outbound link is : url that you have it on your site that redirect to other site. So, while Google PR has become a race for everybody, more and more are getting in bound links to your site. Though your mission should be to get more site to link to yours, and not forget their polar opposites.
So, when you are outbound links to your site, you are reviewing, voting the site you link to. Search engine are working after this method, especially the entire ranking algorithm. So, when two similar websites, in content and design, the site with more links pointed will be considered more important by the search engine.
