Extensible Markup Language. XML describes a class of data objects called XML documents and partially describes the behavior of computer programs that process them. XML is a subset of SGML, and XML documents are conforming SGML documents. XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data is made up of characters (a unit of text), some of which form character data, and some of which form markup. Markup includes tags that provide information about the data, i.e., a description of the structure and content of the document. Character data comprises all the text that is not markup. XML provides a mechanism to impose constraints on the storage layout and logical structure.