Web design literally evolves every day. Technologies that are now integral to your experiencing the internet, were either sparsely used, testing, or just plain nonexistent just 2 years ago! When you log into your Facebook, you can let it sit there and it will update automatically, without you “refreshing” the page as we usually do when we need new information from a page.
XML (eXtensinble markup language) is a complicated subject, but it’s really a system of organizing things. If I want Kate’s favorite colors, to write it in XML I would say something like: <person> <name>Kate</name> <race>Human</race> <favoritecolors> Blue Yellow Green </favoritecolors> </person> Now this is NOT correct XML, it’s merely a rough illustration o what it looks like and what it’s purpose is. You can make your own “fields” like “name” or “race” in the prior example, let me reiterate that I made these names up, they are not “correct” or built in names, I said this “object” (or concept in this case) should be described in XML by it’s name, race and favorite colors. In this format, new technologies can easily interact with each other, and we can write more descriptive programs. You can think of Ajax as a “hole” in your browser. You load Facebook, (or Twitter), and it opens a “hole” in your browser for the constant stream of updates to pour in.