concrete5 makes frequent use of a database for storage of all its persistent data, like Pages, Files, Users, etc... This database is available for developers to make use of as well, and concrete5 provides a full-featured API to access it. The concrete5 database library is based off of Doctrine Database Access Layer which is a very full-featured way to access a database – and is in turn based off of PHP'S PDO.
While concrete5 ensures a MySQL database is present and connected to during installation, most of the calls to the database itself are database-agnostic. That's one of the benefits of using a library like DBAL.