XML

SGML (Standard Generalized Markup Language)

A metalanguage for describing markup languages

XML (Extensible Markup Language)

A simplified subset of SGML which preserves most of the power and richness of SGML but retains all of the commonly used features of SGML, thus removing the more complex features of SGML
It is easier to introduce new elements or additional attributes than in SGML.
General info about XML
XML 1.0 Fourth Edition (W3C recommendation)
XML 1.1 Second Edition (W3C recommendation)

XML Core

General info about XML Core
Namespaces in XML: for separating elements and attributes into different groups
Namespaces in XML 1.0 Second Edition (W3C recommendation)
Namespaces in XML 1.1 Second Edition (W3C recommendation)
XInclude: for merging multiple XML documents into a larger one
XInclude 1.0 Second Edition (W3C recommendation)

XML Schema Languages

XML Schema: for defining the structure of XML documents
XML Schema (1.0) Part 0: Primer Second Edition (W3C recommendation)
XML Schema (1.0) Part 1: Structures Second Edition (W3C recommendation)
XML Schema (1.0) Part 2: Datatypes Second Edition (W3C recommendation)
XML Schema 1.1 Part 1: Structures (W3C working draft)
XML Schema 1.1 Part 2: Datatypes (W3C working draft)
RELAX NG: for defining the structure of XML documents
RELAX NG Tutorial (OASIS committee specification)
RELAX NG Specification (OASIS committee specification)
Schematron: for asserting patterns in XMl documents
Schematron Speficifation) (ISO standard)
Comparing XML Schema Languages

XML Linking

General info about XML Linking
XLink: for inserting elements to create links between resources
XLink 1.0 (W3C recommendation)
XPointer: for specifying paths in URIs
XPointer Framework (W3C recommendation)
XPointer element() Scheme (W3C recommendation)
XPointer xmlns() Scheme (W3C recommendation)
XPointer xpointer() Scheme (W3C working draft)

XML Query

General info about XML Query
XPath: for addressing parts of an XML document
XPath 1.0 (W3C recommendation)
XPath 2.0 (W3C recommendation)
XQuery: for querying XML documents
XQuery 1.0 (W3C recommendation)

XSL (Extensible Stylesheet Language)

General info about XSL
XSLT: for transformaing XML documents
XSLT 1.0 (W3C recommendation)
XSLT 2.0 (W3C recommendation)
XSL per se (also known as XSL-FO): for formatting objects
XSL 1.0 (W3C recommendation)
XSL 1.1 (W3C recommendation)

XForms

XForms: a new generation of forms
XForms 1.0 Second Edition (W3C recommendation)
XForms 1.1 (W3C working draft)

Recommended Offline Reading

Ray, E. T. 2003². Learning XML. Sebastopol, CA: O'Reilly.
van der Vlist, E. 2002. XML Schema. Sebastopol, CA: O'Reilly.
van der Vlist, E. 2004. RELAX NG. Sebastopol, CA: O'Reilly.
van der Vlist, E. 2007. . Sebastopol, CA: O'Reilly.
Simpson, J. E. 2002. XPath and XPointer. Sebastopol, CA: O'Reilly.
Walmsley, P. 2007. XQuery. Sebastopol, CA: O'Reilly.
Fitzgerald, M. 2004. Learning XSLT. Sebastopol, CA: O'Reilly.
Tidwell, T. 2001. XSLT. Sebastopol, CA: O'Reilly.
Pawson, D. 2002. XSL-FO. Sebastopol, CA: O'Reilly.
Dubinko, M. 2003. XForms Essentials. Sebastopol, CA: O'Reilly.