Why use XML?

I have been learning web development in my own time along side doing my CompSci degree. We did a short module in relational database theory and that has come in handy in aiding me in learning how to develop web applications on (W/L)AMP frameworks. I understand XML to be a popular technology but have yet to think of a useful application. Could someone explain to me about AJAX. Is there a constant connection with the server, updating XML in the background at the client side without the page being reloaded? Is that how it works? Can you give me an explain of a useful application?

For data that doesn't need to be secure, it's a low-processing, and easily accessible method.

RSS feeds are XML, and wouldn't work as data in a database, because people would need administrative access to your database. An XML file is just freely accessible, unformatted text.

Just in case you don't know what an RSS feed is, it's just a file that someone can open with an RSS feed reader for quick access to a site's information all in one aggregated place. For example, when a news story is updated on a site, it's appended to the XML file, and when the feed reader refreshes, it reads in that there's a new story.

I'll also note that XML can also be used to display data using a stylesheet, kind of like an all-data web page as well as use a DTD to validate that data upon entry, and if used as a database that the connection isn't persistent -- it should open, read/write, close.

its best for very popular websites, becuase it lowers bandwith strain on servers, becuase it doesnt have to request the whole page, just the info the client wants.