XML (Extensible Markup Languag) is a text-based syntax for markup languages; in other words, a meta markup language. It's very simple and can be used as general data transformation and exchange language. Everybody can define their own XML application with additional rules of what elements and structures are allowed in a certain markup. XHTML itself is one of those XML applications, and one of the most important at that. The rules to define a certain XML are called DTDs (Document Type Definitions) and Schemas. DTDs and Schema's themselves are held in XML.
An XML application could be:
- a language to markup poetry or books
- a language to markup general web documents
- a language to markup a web site structure
- a language to define data transformations
An XML application on its own is not able to do anything; it's just data description, waiting for some application to make use of it by adding functionality, or rendering. The great benefit of XML is that it's quickly becoming an industry wide standard, and parsers to read in the data are developed on almost every system. These parser can provide certain standardized interfaces to the programmer, so that not everyone has to reinvent the wheel.