XML is the Extensible Markup Language, a format for the exchange of structured data.
XML, which is a simplified subset of SGML, was designed as a universal structured document format, but it is general enough to represent arbitrary structured data in a text file. The format of a XML file is given separately in a DocumentTypeDefinition (DTD) or another SchemaLanguageForXML. The XML syntax is quite verbose and not (necessarily) meant to be read by humans. XML imposes a hierarchical tree structure on the data. GraX?, GXL, and RDF all use XML for data exchange. (Quoted from ExchangeFormatBibliography)
There are a number of systems for processing and transforming XML documents
There are also other ExchangeFormats, some of which are based on XML.
-- ArieVanDeursen (thnx, ErnstJanVerhoeven?).