XHTML is the engine which drives the content which is displayed on the screen in it’s rawest form. Basically XHTML describes the sections of a screen. For instance <p>some text</p> tells the browser that some text is a new paragraph. XHTML is very simple and follows strict rules which help you update your website in the future.
XHTML is better than HTML as it limits the tags to only describing what the elements of the nage are and not describing how they should look. CSS should be used to describe how things look. There is no font, b, i and center tags which will save a lot of headaches when you want to update your site or move to another media like the iPhone. The font and center tags are very 1990’s and makes it extremely hard to update a site. For instance you might want to change site wide your font Gill Sans, with XHTML this is 3 lines in CSS compared to 1000’s of changes in HTML. You can have an in-depth read of the differences at http://www.w3.org/TR/xhtml1/