Plone is technology neutral. Plone can interoperate with most relational database systems, both open source and commercial.
However, by default settings, Plone stores its contents in Zope's built-in transactional object database, the ZODB.
ZODB, the Zope Object Database, is a persistent object database system for Python. It provides facilities to write objects to disk automatically, read them in again when they're required by a running program, and cache them.
On the file system, this database is contained in the file Data.fs, normally located in the var directory (for traditional Zope installations) or in var/filestorage directory (for buildout-based Zope installations).