What is the difference between mobile Facebook and Facebook?

Mobile facebook is a subdomain of facebook. Mobile facebook is simplified in the interface, as well as reduced in size, to allow for easy use and fast loading on mobile devices (Where an internet connection is not as fast as a computer.).

Facebook has many different subdomains...

Facebook has signed contracts with some mobile providers to allow free access to facebook on cellphone browsers. Offsite links and photo access, as well as a few other features, however, are not free.