Sub Domains. There is lots of theory out there as to how to use them, and to be honest, most of them have validity. However a few things can be said about how Google looks at them. It should be known that weight / link popularity are not inherited from the parent domain. Sub domains are essentially looked upon as a different site.

A good rule of thumb for sub domains is to use them when you need to separate content, because it might not be relevant with the rest of the site. In my opinion this helps Google understand the true content of your site and will not lead to the dilution of its overall theme.
Another way to use them is promotional marketing purposes, in an attempt to ingrain a message into the user. Perhaps its needed to help the user remember the site, for example a television ad for the new Video by AOL - video.aol.com

Posted in Linking 101 |