Standard Generalized Markup Language (SGML) is a precursor of (GML) General Markup Language. GML was introduced in the 1960s by Charles Gold-fab, Raymond Lari and Edward Mosher working at IBM. The purpose was to help organize the large amount of data and documentation that the company was producing.
In 1986, Tim Berners-Lee created (HTML) Hypertext Markup Language as a subset of SGML by using the rules of Sgml to create the tags used in html. The original purpose of Html was to transfer and display scientific data and physics abstract as tabular text document. It had a definite structure that allowed the scientific community to view, reference and easily share information on separate computers via Hypertext.
However, by the beginning of the 1990s, Hyper Text Transfer Protocol (http) servers began to display the output of Html pages in ways not even Tim Berners-Lee could have imagine. Computer programmers and web developers realized that Hyper Text Transfer Protocol (HTTP) servers could display none tabulatedHtml in a Document Viewer (Browser) using the predefined tags and syntax of Html.
XHTML is more than Html and there are some basic difference in writing Xhtml and Html. Xhtml is an extensible markup language that uses the syntax and rules of XML. This makes XHTML far more restrictive than Html, for it does not allow you to bend or break the rules. (XHTML) Hypertext Extensible Markup language can be extended with modules that can do complex mathematical calculations and even draw graphical images.
XHTML is the successor to HTML which is no longer being developed. With ongoing refinement, it can be considered as the "current version" of HTML, but yet a separate and parallel standard. The W3C continues to recommend the use of either XHTML 1.1, XHTML 1.0, or HTML 4.01 for web publishing.
XML (Extensible Markup Language) was created as a subset of Sgml. It's actually a lighter or modified version of SGML. Today, Xml is the new standard that any markup language must adhere to. It is a general markup language, or, we can call it "the Father of all languages" that can be used to describe other markup languages including Html.
Xml is not entirely about webpages, it is much more than the web. Webpages written in Xhtml can easily interact with Xml which can be used to represent any kind of information on all types of computers, hand held devices and cellphones. In order for webmasters to fully exploit these capabilities when writing Html webpages, the W3C once again provided a new and lighter version of html, called XHTML (Extensible Hypertext Markup Language).
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