Introduction to Databases
The principle of interactivity is clearly seen in databases and their use online. The idea of a database is simple: where plain websites are entirely static (they display the same content every time you look at them, unless the have been manually updated), data-driven websites is one of the things that makes a website dynamic, with new, fresh, content. Essentially, what displays on the page is determined in some part by what's in the database. (Greenspan)
You might be familiar with how a basic webpage works. HTML code that is already written is sent to your browser. Your browser translates this code into a webpage which you can see. With a webpage that uses a database, all or some of the HTML is generated by the data in database, adding an extra step to the process, but meaning that you'll see new data fresh from the database. When the maintainer of the website wants to update a page, he or she manually or automatically (usually the latter) changes the data in the database. The next time you view the page, you'll interact with new data.
How is this interactivity, you might ask. Simple: the website provides you with fresh data from the database, and gives you the opportunity to change the database in most cases (such as when you rank an article or video). Rather than just presenting one, stale page of HTML, it presents a fresh page of generated HTML that represents real-time data. When you perform an action, the data will often update, and, the information displayed. (Chappel)
Most moderately complex websites use some form of a database. A simple example is an e-commerce site. If you view an item for sale on an e-commerce site, the website will check to make sure the item is in stock before the present a "Checkout" button. If the item isn't in stock, it won't let you buy it, but will rather display a message informing you of this. Notice how sites like Amazon.com and Newegg.com do this the next time your purchase something. Also, most user interactivity connects back to a database at some point. All of those comments and stories from users on Digg, tags on a Flickr photo, or articles on Wikipedia are stored in a database of some sort.