XHTML 1.0 is an XML-based reimplementation of the SGML-based HTML 4.0 specification. Many HTML files are not WellFormed XHTML, but can usually be converted relatively simply to XHTML 1.0. Those that can't usually have UndefinedSemantics anyway.
XHTML 1.1 is much stricter than the 1.0 specification and a lot closer to the spirit of XML so it can finally bring many cool features to the table (no pun intended). By being XML it can take advantage of the semantics of other XML languages discard much of the therefor redundant features of HTML (and XHTML 1.0). F.ex, using XLink? semantics, images can be included in a documented using f.ex the p (paragraph) tag, rendering the img tag superfluous. (In fact, pretty much any tag can be used for the purpose.)