XHTML (Extensible HyperText Markup Language) is a reformulation of HTML 4.0 by definition.   It is an application of XML 1.0 and a family of current and future document types and modules that reproduce subset and extend HTML 4.0.  XHTML family document types are XML based, therefore, they can be designed to interact and work in conjunction with XML-based user agents.

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.

XHTML 1.0 specification is the first document type in the XHTML family. It is a reformulation of the three HTML 4.0 document types as applications of XML 1.0. . If some simple guidelines are followed, developers who migrate their content to XHTML 1.0 will realize the following benefits.

Xhtml 1.0 is specified in three "flavors." You specify which of these variants you are using by inserting a line at the beginning of the document. For example, the HTML for this document starts with a line which says that it is using XHTML 1.0 transition.

XHTML 1.0
XHTML 1.0 Strict is very unforgiving.  Use this this together with Cascading Style Sheet(CSS) to get the font, color, and layout effects you want. This Doctype is for when you want really clean structural mark-up, free of any depricated tags associated with content layout.

XHTML 1.0 Transitional.
If you are writing Web pages for the general public to access, you might want to use this flavor of XHTML 1.0. The idea is to take advantage of XHTML features including style sheets but nonetheless to make small adjustments to your markup for the benefit of those viewing your pages with older browsers which can't understand style sheets. These include using the body element with bgcolor, text and link attributes.

XHTML 1.0 Frameset.
Use this when you want to use Frames to partition the browser window into two or more frames.

The complete XHTML 1.0 specification is available in English in several formats, including HTML, PostScript and PDF at .W3C HTML

If you want to validate the document, the tool used to do so automatically detect which variant you are using.  Each variant has its own (DTD) Document Type Definition which sets out the rules and regulations for using HTML in a succinct and definitive manner.