Hey Pam -
I can answer part of your question. The difference between HTML and (X)HTML is just the syntax used. (X) is for the extensible markup language, compared to HTML which is just known as markup language. In other words, it's just an upgrade and works better with various software. Just makes it more predictable to how its handled. Just know that there are enough differences between HTML & (X)HTML, because of how older browsers handle empty tags with (X)HTML, and perhaps other things that I'm not aware of. For instance...in (X)HTML, if we want to insert a break we would insert a single tag <br/>...but because older browsers don't know about (X)HTML we have to create the tag like this...<br />, because the older browsers don't pay attention to the space.
The doc type declaration is what tells browsers how they are to interpret the document. To get a better idea of what it all means try this link. Good resource - but not to be confused with the W3C itself...just as a side note.
http://www.w3schools.com/tags/tag_doctype.asp
Also...to go to "the" source at W3C - Look here
http://www.w3.org/QA/Tips/Doctype and scroll down to the middle of that page and click on the articles there.
Hope this helps a little. Sorry didn't have anything to offer about HTML5.