XHTML (a merger of XML and HTML) is an updated HTML standard introduced to help tidy up all the developments and additions to original HTML. Over the years, different browsers added features and offered increased functionality, which all had their own proprietary manifestation in HTML. This led to a mish mash of tags and markup that often led to web authors putting in duplicate attributes and code in order to make sure that a page looked the same in different browsers browsers.
XHTML is a unifying standard from the W3C that brings the XML benefits of easy validation and troubleshooting to HTML, which not only helps developers but also makes the pages easier to read for programs such as mobile browsers.
There are three variations of XHTML, the most common of which is Transitional. XHTML Transitional is a stepping stone from the deprecated HTML standardsL, and as such is fairly permissive, allowing many old and proprietary attributes, although Transitional stipulates that all tags must have both opening and a closing markup and be correctly nested. XHTML Strict is a more rigourous standard that removes all the ’chaff’ gathered by HTML not ruled out in Transitional, and is what web authors should strive toward. Finally, XHTML Frameset is similar to XHTML Transitional, but allows for the use of frames.