XML stands for eXtensible Markup Language.
In simple terms, it's a way of describing data. It's a textual way of structurally representing any kind of information.
The main purpose of this language is to provide a unified way of exchanging information between different resource providers, both offline and online ones.
To someone who's never seen it, XML might see to be overly complicated, but the beauty of it is that many programming languages support it so extensively, that you wouldn't have to spend time parsing it or writing any routines for managing your XML data – it's all been written before, so you can just concentrate on your content – the valuable information you have to share or exchange with others.