Introduction to XML
The eXtensible Markup Language (XML) is a specification introduced by the World Wide Web Consortium (W3C) for identifying and organizing elements of information in electronic documents. XML is the result of scaling down the Standard Generalised Markup Language or SGML for short an earlier standard for document representation, developed and standardised by the International Organisation for Standardisation (ISO) in 1986.

In XML, data elements are designated using pairs of tags (a start tag and an end tag) that surround the data being “tagged”. Tags are arbitrary identifiers enclosed in angle brackets, e.g., <message>this is a message</message>. In the terminology of XML, data being tagged in this manner are called elements. An element is a named entity; its name is the tag used to annotate it. In the above example, message is an element name. An element may not only consist of textual data, but it can comprise attributes and other elements as well.

Obviously, an XML document that does not specify a DTD (a standalone document in the terminology of XML) cannot be validated, since there is nothing to validate against to. Document type definitions have played an important role in XML, although the weakness of this language had been pointed out since the very beginning. It seems that DTD fall into decline while XML Schema is going to take on from now on.

Core XML Standards
The W3C have chosen to publish a minimal specification for XML, while in the same time formed a number of working groups in order to provide specifications for issues that although crucial for the evolution of XML and its adoption for large scale applications, are not covered in. Thus since 1998, when was first published, a number of specifications related to XML have appeared. In order to give a more clear depiction of the XML landscape, a short look on the most important of these specifications is in place.

Tools for XML processing
There are currently a lot of XML tools that seek to facilitate the use of XML, bridging business applications with the use of XML by visualising the processes of the work. The most important ones are presented below
ALTOVAs XML SPY
HPs Bluestone Visual-XML
IBMs XML TOOLS
IBMs VISUAL DTD
IBMs VISUAL XML TRANSFORMATION
IBMs VISUAL XML CREATION
IBMs VISUAL XML QUERY
INSIGHTs XMLMATE
SOFTQUADs XMETAL