[thelist] What is DOCTYPE?

[John Craft]

I've seen mentioned on this newsletter various problems with including
DOCTYPE in HTML, but I don't really understand why that is.  I've also
recently read somewhere (the site escapes me) a workaround that included not
putting in the full URL in the DOCTYPE declaration.  Could someone please
explain in more detail what DOCTYPE does and when and when not to use it?
Does using DOCTYPE automatically validate your code with the W3?