XHTML is the reformulation of HTML as an application of XML
If you are familiar with html 4 you should have no problems with learning
XHTML. It is the first step towards modularization based on the xml model.
It provides the bridge for web designers to enter the web of the future, while
maintaining compliance with today's browsers. It looks very much like html
4, with a few notable exceptions.
One of the first things you should notice in the above example is the (DTD) Doc Type Declaration. When an XHTML document is created, the DTD to which it conforms is declared in a Document Type Declaration at the top of the document. Each DTD may be recognized by a unique label called a formal public identifier, or FPI. The literal (ie., quoted) text following the word "PUBLIC" is an FPI referring to the W3C's XHTML 1.0 DTD. XHTML 1.0 specifies three XML document types that correspond to the three HTML 4.0 DTD's: Strict, Transitional, and Frameset.
Use this when you want really clean markup, free of presentational clutter and add a Cascading Style Sheet to control your presentation.
Use this when you need to take advantage of HTML's presentational features if you are concerned that your viewers may not have the latest browsers that understand Cascading Style Sheets.
Use this when you want to use Frames to partition the browser window into two or more frames.