Okay so I made it to page 8 in the book just fine, I have an okay knowledge on programming but I have never learned any web languages so my questions are going to be very beginner.

Is document type declaration a very common practice? I now it should be in the beginning of my document but should it be the absolute first line of the document? If so, What sort of tag do I give it?

Another thing I'm a Little cloudy on is the difference between html, xhtml, xml, html5, and so on....... This book says xhtml is the latest version of the html language, is this true?

Thanks for the help!!!