Differences Between XHTML, HTML, and XML

XHTML is HTML 4.0 Written in XML Format
On January 26, 2000, the W3C announced that HTML 4 has been reformulated into XHTML 1.0. XHTML is HTML 4 written as an XML application.

There are two primary parts to XHTML:

HTML 4
HTML 4 is a markup language used for displaying text and documents across different platforms and machines. It was originally intended for a very specific audience, and has expanded to include hypertext, multimedia, as well as the style of the documents displayed.

XML
XML is an extensible markup language that was developed to retain the flexibility and power of HTML while reducing most of the complexity.

XHTML combines the flexibility of HTML with the extensibility of XML.

Benefits of XHTML

A painless transition to more advanced technology

The web is moving to XML, a powerfully enabling technology. Writing well–formed, valid XHTML pages is the easiest way to begin this transition. All it takes is learning a few simple rules of XHTML markup.
Cleaner, more logical markup
XHTML brings uniformity to document structure. The rules of XHTML help restore the structural integrity of documents that was lost during the web’s rapid commercial expansion between 1994 and 2001. This is critical for large organizations such as ours, whose web pages must interface with logically–marked–up documents in legacy systems and databases.

Increased interoperability
Unlike old–style HTML pages, valid, well–formed XHTML documents can easily be “transported” to wireless devices, Braille readers and other specialized web environments. Moreover, XHTML’s insistence on clean, rule–based markup helps us avoid the kind of errors that can make web pages fail even in traditional browsers like Microsoft Internet Explorer, Netscape Navigator, and Opera Software’s Opera browser.

Greater accessibility
Because they follow strict rules and avoid non–standard markup, well–authored XHTML pages are more accessible than old–school HTML pages, helping the library comply with U.S. laws and accessibility guidelines.

What is XHTML ?

XHTML is a family of current and future document types and modules that reproduce, subset, and extend HTML 4 [HTML4]. XHTML family document types are XML based, and ultimately are designed to work in conjunction with XML-based user agents. The details of this family and its evolution are discussed in more detail in [<XHTMLMOD].
XHTML 1.0 (this specification) is the first document type in the XHTML family. It is a reformulation of the three HTML 4 document types as applications of XML 1.0 [XML]. It is intended to be used as a language for content that is both XML-conforming and, if some simple guidelines are followed, operates in HTML 4 conforming user agents. Developers who migrate their content to XHTML 1.0 will realize the following benefits:
XHTML documents are XML conforming. As such, they are readily viewed, edited, and validated with standard XML tools.
XHTML documents can be written to operate as well or better than they did before in existing HTML 4-conforming user agents as well as in new, XHTML 1.0 conforming user agents.
XHTML documents can utilize applications (e.g. scripts and applets) that rely upon either the HTML Document Object Model or the XML Document Object Model [DOM].
As the XHTML family evolves, documents conforming to XHTML 1.0 will be more likely to interoperate within and among various XHTML environments.
The XHTML family is the next step in the evolution of the Internet. By migrating to XHTML today, content developers can enter the XML world with all of its attendant benefits, while still remaining confident in their content's backward and future compatibility.