Ok i am going to try and clear up some of this.
HTML and XHTML technically are differant lanaguages.
Xhtml is a much cleaner langauge and makes the coder code better meaning their pages are more complinet and more cross browser compat etc..
the most simple thing is that all it requires extra is to close all tags. so say with html 4.01 its valid to have <br> in XHTML you have to close it so <br /> and <img /> etc also require quotes like <img src="src.jpg" />
and as the strictness of xhtml some elements and tags are removed such as the target in anything above transitional and a few others.