the Extensible HyperText Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax [Wikipedia].
even though XHTML is a relatively old standard (it was officially recommended by the W3C in January 2000), it has recently become significant for designing web pages. Unlike traditional HTML, XHTML strictly enforces the standards of XML, which in turn is a very important information management standard.
XML documents are desirable because they can be 'described' or displayed differently by different systems, such as browsers, data bases, spreadsheets, and other software applications. Essentially it is organized information without any rules about usage or display. So, what may look like a web page in a web browser can appear differently in a phone browser or screen reader without having to recode it.
the benefit of a document that is written in clean XHTML over a 'tag soup' web page is that it eliminates many of the portability and viewing issues that have plagued older web sites, and it removes the opportunity to 'lazy code' a web site.
The web has been a rapid evolution of technology, and as such has dragged along many undesirable legacy bits and pieces. Valid XHTML minimizes many of these issues. By enforcing a much cleaner standard in your design, your web site will have a longer life and wider viewing audience.