An online database can be used for a multitude of purposes, from storing news articles to recording customer form submissions.
A more obvious use for an online database is to display products that you wish to allow users to purchase via a shopping cart, or to display properties if you are an estate/letting agents
Although not all websites may appear to be running from a database, they could well be - a database can also allow you to to control your website via the implementation of a Content Management System (CMS).
An online database usually comes hand-in-hand with an Administration Area which, as the name would suggest, is used to 'administor' the database.
Some examples of database-driven websites are...