Eli didn't mention pictures and music. Those can also be stored in a database. Anything that can be stored in a digital format can be kept.
If this isn't just a homework question, then you should research the how data is represented. Essentially, it's just bits -- ones and zeroes. The same 01000001 can be interpreted as 65, or the letter "A" or it can be considered a level of red or blue or green of a pixel in a picture.
So, the other thing a database can contain is "metadata" which is data about data, which would keep track of how those 8 bits should be interpreted.
Huge amount of data in a sorted or specific order is placed inside the database. It can be anything names, locations, numbers, addresses, etc. I would add to the answer of EddieJ that metadata is a data about the data. It describes how,when and by whom a certain form of data was collected and inserted into the database.
A database holds all the data, literally. It holds passwords, usernames, credit card numbers, locations, etc. It can hold administrative accounts, cookies (encrypted passwords), and may even connect to a separate server used by administrators to work on the website.
Data, basically. What exactly depends on the database.
Common SQL databases typically hold numbers, text, dates etc. and let you search for those - "all records with user=joe", for example. But they can also hold binary data (BLOB - binary large object) so the database can hold images or audio etc.
Data.
Anything you want.
