XML是eXtensible Markup Language，即可扩展标记语言，由SGML简化而来，是许多置标语言（例如XMLs/RDF/RDFs/OWL等）的元语言。

XMLS指XML Schema，即XML模式，用于定义XML词表和结构用于机器识别和共享信息的定义语言。

Express shared vocabularies and allow machines to carry out rules made by people. They provide a means for defining the structure, content and semantics of XML documents. [W3C consortium "XML schema"] http://www.w3.org/XML/Schema

XML Schema is a language for describing the structure and constraining the contents of XML documents