Since Cognos 8.2 there is a new check option to install, called "Content Store Database".
Checking this option installs Apache Derby - an open source DB - to use as the content store database. Normally you do not install this but it provides a fully functional Cognos content store for demo & POC applications without needing a major database installed. After installing, just configure, start services, and you're up and running with a functional content store.
Apache Derby is not as robust as any of the supported traditional RDBMS products, that serve as the content store in a production environment!