XML is a small subset of SGML allowing the definition of own tag-based DataFormats.
Formats like the ChannelDefinitionFormat, the OpenSoftwareDistributionFormat or the MetaDataResourceDescriptionFramework are created with XML.
Please do not confuse this with HTML, which is an application of SGML, not a subset.
You can access the most recent information on XML at The W3C Specification of XML
Other sites:
http://www.xmlinfo.com/
http://www.xml.com/
http://www.textuality.com/xml
http://www.jclark.com/xml/
http://www.sil.org/sgml/xml.html
http://openxml.org/