title: Compare/contrast HTML, XHTML, XML, and HTML5
body: <p>Ok, so I'm not quite sure the difference between these languages.  Could someone clarify?  I know that XML has user-defined tag and html is pre-defined, but thats basically the extent of my knowledge.</p>
  
  <p>I know that HTML5 is supposed to replace HTML, but wasn't XML supposed to do that as well?  Basically, which languages here are a substitute for the other, and which complement?  Does XML replace XHTML?</p>