For a general overview of databases and how they are used in MediaPortal, see:

In Configuration you may choose to scan your disk drives and index all your media files in a database, particularly for music or videos.  Images and detailed information about each item, such as an music artist or a movie, will be automatically downloaded, stored in your database, and then displayed using any database view.

You may use several database views, such as:

You can also define your own views for Music or Videos, as well as customize them.  See Configuration