TYPES Of Mark Up Langugae:-

Tex
Another major publishing standard is TeX, created and continuously refined by Donald Knuth in the 1970s and 80s. TeX concentrated on detailed layout of text and font descriptions in order to typeset mathematical books in professional quality. This required Knuth to spend considerable time investigating the art of typesetting.
Scribe
The first language to make a clear and clean distinction between structure and presentation was certainly Scribe, developed by Brian Reid and described in his doctoral thesis in 1980. Scribe was revolutionary in a number of ways, not least that it introduced the idea of styles separated from the marked up document, and of a grammar controlling the usage of descriptive elements. Scribe influenced the development of Generalized Markup Language (later SGML) and is a direct ancestor to HTML and LaTeX.

GML
In the early 1980s, the idea that markup should be focused on the structural aspects of a document and leave the visual presentation of that structure to the interpreter led to the creation of SGML. The language was developed by a committee chaired by Goldfarb. It incorporated ideas from many different sources, including Tunnicliffe's project, GenCode. Sharon Adler, Anders Berglund, and James A. Marke were also key members of the SGML committee.

SGML
SGML specified a syntax for including the markup in documents, as well as one for separately describing what tags were allowed, and where (the Document Type Definition (DTD) or schema). This allowed authors to create and use any markup they wished, selecting tags that made the most sense to them and were named in their own natural languages. Thus, SGML is properly a meta-language, and many particular markup languages are derived from it. From the late 80s on, most substantial new markup languages have been based on SGML system, including for example TEI and DocBook. SGML was promulgated as an International Standard by International Organization for Standardization, ISO 8879, in 1986.

HTML
By 1991, it appeared to many that SGML would be limited to commercial and data-based applications while WYSIWYG tools (which stored documents in proprietary binary formats) would suffice for other document processing applications.

The situation changed when Sir Tim Berners-Lee, learning of SGML from co-worker Anders Berglund and others at CERN, used SGML syntax to create HTML. HTML resembles other SGML-based tag languages, although it began as simpler than most and a formal DTD was not developed until later. DeRose argues that HTML's use of descriptive markup (and SGML in particular) was a major factor in the success of the Web, because of the flexibility and extensibility that it enabled (other factors include the notion of URLs and the free distribution of browsers). HTML is quite likely the most used markup language in the world today.

XML
Another, newer, markup language that is now widely used is XML (Extensible Markup Language). XML was developed by the World Wide Web Consortium, in a committee created and chaired by Jon Bosak. The main purpose of XML was to simplify SGML by focusing on a particular problem — documents on the Internet. XML remains a meta-language like SGML, allowing users to create any tags needed (hence "extensible") and then describing those tags and their permitted uses.

XML adoption was helped because every XML document can also be written in such a way that it is also an SGML document, and existing SGML users and software could switch to XML fairly easily. However, XML eliminated many of the more complex and human-oriented features of SGML to simplify implementation (while increasing markup size and reducing readability and editability). Other improvements rectified some SGML problems in international settings, and made it possible to parse and interpret document hierarchy even if no DTC is available.

XHTML
Since January 2000 all W3C Recommendations for HTML have been based on XML rather than SGML, using the abbreviation XHTML (Extensible HyperText Markup Language). The language specification requires that XHTML Web documents must be well-formed XML documents – this allows for more rigorous and robust documents while using tags familiar from HTML.

Difference Between HTML and XHTML
One of the most noticeable differences between HTML and XHTML is the rule that all tags must be closed: empty HTML tags such as
must either be closed with a regular end-tag, or replaced by a special form:
(the space before the '/' on the end tag is optional, but frequently used because it enables some pre-XML Web browsers, and SGML parsers, to accept the tag). Another is that all attribute values in tags must be quoted. Finally, all tag and attribute names must be lowercase in order to be valid; HTML, on the other hand, was case-insensitive.

Feartures
A common feature of many markup languages is that they intermix the text of a document with markup instructions in the same data stream or file. This is not necessary; it is possible to isolate markup from text content, using pointers, offsets, IDs, or other methods to co-ordinate the two. Such "standoff markup" is typical for the internal representations programs use to work with marked-up documents. However, embedded or "inline" markup is much more common elsewhere. Here, for example, is a small section of text marked up in HTML:

Uses And Applications
While the idea of markup language originated with text documents, there is an increasing usage of markup languages in areas like
1) vector graphics
2) web services
3) content syndication 4)
user interfaces.
Most of these are XML applications because it is a well-defined and extensible language. The use of XML has also led to the possibility of combining multiple markup languages into a single profile, like XHTML+SMIL and XHTML+MathML+SVG