Standard Generalized Markup Language
SGML is an acronym for Extensible or (Xtensible Markup Language). XML can be described as a meta-language that enables a general availability and interchange of information that is structured according to its content. XML is a subset or child of (SGML) "Standard Genralized Markup Language" the mother of all languages.
XML Is A Meta-Language
Xml can be used to describe and generate other language markup that are interoperable with any kind of application in various presentation for different target groups and purposes. Given its flexibility, it is perfectly suited for marking up information of any kind.
XML & HTML Are Not The Same.
However, since they derived from SGML, they are similar but not identical. Get this straight, Xml is not an improvement of HTML, the concept and engineering of Xml is an initiative and recommendation of the World Wide Web Consortium (W3C) (www.w3.org) intended to create an open-ended markup language that could easily accommodate future expansions and addition.
Differences Between SGML and XML
The differences between SGML and XML are distinguished in size, weight and portability. The later arose from a mandate to develop a ultra-light meta-language especially for the needs of internet content developers and to promote a fast establishment of this language on the (WWW) world wide Web.
The goal and objective of the initiative is to bring some of SGML capabilities and advanced features to the web via XML. Well formed Xml documents allows flexibility instead of fixed structure: Whereas HTML provides only a fixed amount of elements for heterogeneous information, XML is better able to generate elements which are tailor-made to particular information types.
One good example is an SGML document which is structured according to the pattern of the DTD, dependent on the industry. Usually, essential information is lost in Web publishing since HTML does not provide sufficient modes of expression. XML makes this information "Web-capable" and ready for deployment across all platforms.
Access to information instead of only to layout:
HTML offers limited possibilities for structuring documents according to their content. It is mainly a layout-orientated language for the display of documents. Targeted retrieval, for example, is not possible with HTML, but it is with XML and SGML data (e.g., search for the last name of an author).
Just like in SGML, it is possible to parse documents. The XML DTD can be formulated in a way so that the basic requirements on the structure of the documents (e.g., sequence, existence of particular elements) is automatically verifiable (via an XML parser).
The language capacity of XML is limited, and therefore the development of applications for XML is less complex than for SGML. The dissemination of XML for Web publishing is thus favored.
DTD not necessary:
XML documents can be used without a DTD (Document Type Definition). Thus, XML can be used for structuring as regards content and as a pure presentation tool. It is possible to use it according to its purpose. For presenting and downloading data, the document can be used separately in order to facilitate and accelerate processing. If the structure of the document is relevant and has to be controllable, then the document is transported together with its DTD.
If the DTD is missing, XML documents have to be well formed, that is their structure has to fulfill specific pre-conditions in order to be able to be interpreted and processed correctly in all applications. The most important criteria of well formedness: