SilverStripe uses MySQL to store most of its data yes, although one of our Summer of Code guys is working on Postgres and MS SQL server, via the PDO database abstraction layer.
Have you gone through the four tutorials? You'll notice SilverStripe has more of an emphasis on being a coding framework (like ruby on rails) than Joomla, although it obviously has a the idea of a pre-built CMS system, with modules, themes, widgets.