Trouble understanding XML

I'm a Visual Basic programmer. I've read through a few beginner tutorials on XML.

My main question is: is XML just a data holder for programs to use? Or can it do anything on it's own?

I keep feeling that I'm missing some concept. Perhaps I'm just thinking that there's more to XML than there is.

Here's what I understand, tell me if I'm wrong:

Example:
I could write a VB program that would output data to an XML file. Then I could get that data using...Flash ActionScript (let's say because that's the only other language I'm proficient at, and I know it can use XML); where normally it'd be impossible (or at least hard) for them to send data back and forth.

Is this correct? And is there anything else XML is used for, in and of itself?

I'm new to XML too, but it's my understanding that XML is used to store and describe data. And as you pointed out, one of the advantages is that the data is easily used by multiple languages accross multiple platforms.

It's also my understanding that you can use XML by itself, similarly to HTML, but only after an extra step. Since XML only describes data, you have to format the data. You can use Cascading StyleSheets, custom DocType Definitions, XSL (which seems to be a language itself), or XSD (XML Schemas). I believe the current recommended approach is XSD.