How do I start learning to use XML?

Hello, and I’ve been Actionscripting for quite a while now, and a lot of the sources I see and books I read talk about something called XML. I understand that its a formatting language used to communicated with servers, and the syntax looks like html.

I have a few books on AS, but I don’t really get the XML stuff. Is the idea that I use an XML segment of code in my Flash to import in a document to flash at runtime? Or is it that I can load variables from an external document?

I don’t have my own server but I do have the ability to upload files to servers, so I could probably make a file that can communicate with an SWF, and I’m told XML is the way…

So does anyone know a good way to learn about XML, and what is actually is and can do?