Martin Bryan. Introducing the Extensible Markup Language (XML)
XML is a subset of SGTML. Unlike other markup languages, XML indetifies every part of a document. XML operates on the foundation that documents are a series of entities and each entitie contains one or more "logical elements." Each of the elements has attributes. "XML provides a formal syntax for describing the relationships between the entities, elements and attributes that make up an XML document, which can be used to tell the computer how it can recognize the component parts of each document."
Uche Ogbuji. A survey of XML standards: Part 1. January 2004.
The document consist of a brief history of XML (start in Unicode) and provides several links to XML tutorials.
Extending your Markup: a XML tutorial by Andre Bergholz
Good XML documents start with a prologue and contain one element.
DTD-defines the structure of XML documents. DTD lets a user specify the tags used, order of tags, and the attributes associated with each tag.
Developments in XML: Resource Description Framework (RDF)-integrates metadata activities.
Document Object Model (DOM)-allows programs to access and update content.
XML Schema Tutorial
XML Schema -and XML alternative to DTD. Language known as XSD.