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Extensible Markup Language (XML) is a simple, very flexible text format derived from SGML (ISO 8879). XML documents fall into two broad categories: data-centric and document-centric. Data-centric documents are those where XML is used as a data transport. Examples include sales orders, patient records, directory entries, and metadata records.
One significant use of data-centric XML is for manifests (lists) of digital content; another is for metadata embedded into digital content files. Document-centric documents are those in which XML is used for its SGML-like capabilities, reflecting the structure of particular classes of documents, such as books with chapters, user manuals, newsfeeds and articles incorporating explicit metadata in addition to the text.
An XML document's markup structure can be defined by a schema language and validated against a definition in that language. The initial, and as of 2008, most widely used schema languages are the Document Type Definition (DTD) language and W3C XML Schema. Other schema languages exist, including RDF and RELAX-NG.
Perfection is achieved not when there is nothing more to add, but rather when there is nothing more to take away.
Antoine de Saint-Exupery
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