I had a mini-land mark happening recently. I was creating a web page that allowed a user to scroll through images of a photo library. Now normally this wouldn't be that big of a deal because they're just new links/pages. However, this page was different because there is music playing in the background. I couldn't have the entire page refresh because then the song would start all over. Lame.
There are a couple ways to tackle this problem and I decided to try doing it with Ajax. Basically, Ajax allows you to update only parts of a page without reloading the entire thing. It sends a simple command to the server which returns only that part. This has two benefits. First, once the page is loaded, updating information on the page is really fast. Second, it allows for cool visual effects including, like I'm doing, letting things continue to run while other parts change. If you use Gmail, many parts use Ajax.