What is the difference between HTML and XHTML?

How can I tell if a code includes XHTML?

As I understand it XHTML is just well written HTML, so isn't it possible that if you know HTML you maybe accidentally writing XHTML just by using good structure and syntax? So learning to use XHTML doesn't involve much then?

Is there any difference visually to the users of the webpage?

I've also heard that HTML 4.0 or something is the same as XHTML, is this true?

James