XML (Extensible Markup Language) is a set of rules for encoding documents in both human-readable and machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C
XML (Extensible Markup Language) is strictly speaking a meta-language for defining other markup languages.
XML is derived from, and intended to be a simpler variant of, Standard Generalized Markup Language (SGML).
Markup languages are defined by means of XML Schema, as opposed to the Document Type Definiton (DTD) of SGML.
An XML document that conforms to a particular XML schema, is known as an instance, or instance document.
Two phrases often encountered in relation to XML documents are, well formed and valid.
A well formed XML document is one that follows the basic XML encoding rules, a valid XML document is one that conforms to all the additional rules for that XML language as defined by the XML schema.
Common XML languages relevant to GIS:
- Keyhole Markup Language (KML)
- Geography Markup Language (GML)
- Styled Layer Descriptor (SLD)