There are vast amounts of data held in the many online databases on the web, and much of this data is unsearchable, because the information is only produced in response to the web user's interactions which search engine crawlers are unable to simulate. This data, often called the Invisible Web, consists of a great deal of dynamic data such as share and commodity prices, product information and specifications.

Whilst some of this information is useless to the search engine (rapidly changing data would be outdated as soon as it reached the search engine's cache), there is value in searching other parts. Gradually, web database designers are providing searchable static indexes to the useful parts of their data. There are also search engines that catalogue only this dynamic data, such as Kelkoo or Shopping.com which both crawl the web for latest pricing information.

There are also ways in which we can interactively add data to the web, usually through databases. Most database systems have some capability to display, modify edit and add to their data repository through a web-based interface. An example might be an online bank, which would not only allow you to view statements, but to update standing orders, make transfers and even pay bills though the web interface. In making any of these transactions, you are accessing a database of your account details.

Perhaps more obviously, databases are prevalent wherever there is a large body of user-created data. In recent years, online media-sharing sites such as Flickr for photographs, or YouTube for home videos, have become extremely popular, and are very good at storing large amounts of data very efficiently. Even social-networking sites such as Facebook have a database as their storage medium. These types of applications have come to be known as Web 2.0, highlighting a paradigm-shift from static, authored/edited pages to dynamic, user-generated, collaborative pages.