New to XML

I am new to XML and hopefully you guys can answer a few questions…

First off, what is the advantage of using XML as opposed to just using an external text file?

I know in XML you make your own tags and refer to the tags to get information but how can this be implemented in Flash. I want to make it so my client can easily update content so is there a way to make them a flash interface that will alow them to change content without have to open the actually XML file. I mean that would be just as hard to them as HTML. Is there any tutorial you guys know that shows how to make it so the client can you a Flash interface to change the content of their site? Thanks