As such, they are readily viewed, edited, and validated with standard XML tools. XML describes a class of data objects called XML documents and partially describes the behavior of computer programs which process them. By construction, 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, some of which form character data, and some of which form markup.
Markup encodes a description of the document's storage layout and logical structure.
XML provides a mechanism to impose constraints on the storage layout and logical structure. A software module called an XML processor is used to read XML documents and provide access to their content and structure. It is assumed that an XML processor is doing its work on behalf of another module, called the application. The meaning of the signature may be extensible by a set of semantics specified separately. An XSL stylesheet specifies the presentation of a class of XML documents by describing how an instance of the class is transformed into an XML document that uses the formatting vocabulary.