XML - Extensible Markup Language - The Extensible Markup Language (XML) is the universal format for structured documents and data on the Web.
XML is primarily intended to meet the requirements of large-scale Web content providers for industry-specific markup, vendor-neutral data exchange, media-independent publishing, one-on-one marketing, workflow management in collaborative authoring environments, and the processing of Web documents by intelligent clients. It is also expected to find use in certain metadata applications. XML is fully internationalized for both European and Asian languages, with all conforming processors required to support the Unicode character set in both its UTF-8 and UTF-16 encodings. The language is designed for the quickest possible client-side processing consistent with its primary purpose as an electronic publishing and data interchange format.
SGML - Standard Generalized Markup Language – It is an ISO Standard metalanguage in which one can define markup languages for documents. SGML is not in itself a document language, but a description of how to specify one.