What does data partition in nandroid backup contain?

I've been trying to find information on what's included in the **data partition** when doing a nandroid backup. I couldn't quite figure if it includes: + Media files (images, videos, music) + User apps + Configs + System Apps + Configs + Whatever else... Reason being, I wondered if I could just backup system and boot: + The media files are synced using rsync. + The system apps are updated anyway when flashing new ROM. + The user apps are backed up using titanium or whatever. This could save some space of my backups, since I don't have much to spare - having no external sd slot and only 16gb of storage...