i have heard of all of the following:
HTML DHTML SHTML XHTML
so what's the differents between them all? and what do they all do?
p.s i no and understand html

shtml is what you call your file when you use SSI (http://www.lissaexplains.com/html6.shtml)

dhtml is dynamic html, which can be style sheet things or menus and stuff. More here (http://www.yourhtmlsource.com/javascript/dhtmlexplained.html)

xhtml is : extensible html, just a different way of doing html. More here (http://www.w3schools.com/xhtml/xhtml_html.asp)

mm. while we're on the subject, neopets uses the extenion .PHTML what is that?

oo never seen that one but this is what i found:

http://www.computeruser.com/resources/dictionary/definition.html?lookup=6724

and

http://archive.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/thrift/PPI.html

.phtml can also be a PHP page, and is just an alternative to .php, .php3, or .php4

interesting... confusing but interesting...

Why have all thes (x)Html and not just a big one

They all do different things, in some cases they're completely different languages, so you need different extensions.