XML is a meta-language.
A meta-language is a language that's used to define other languages. You can use XML for instance to define a language like WML.
XML is a smaller version of SGML. It's easy to master and that's a major advantage compared to SGML which is a very complex meta-language.
Define data structures
Make these structures platform independent
Process XML defined data automatically
Define your own tags
Define how your data is shown. To show data, you need other techniques.