Doctypes

There are three kinds of doctypes: Strict, Tansitional, and Framset. I will list them bellow for HTML 4.01 and XHTML 1.0.
ALL WEB PAGES SHOULD HAVE A DOCTYPE
"DOCTYPES are also essential to the proper rendering and functioning of web documents in compliant browsers like Mozilla, IE5/Mac, and IE6/Win."
~alistapart.com

(X)HTML Strict: you can't use deprecated tags(basefont tag, center tag, font tag, strike tag or s tag, u tag, applet tag, dir tag, isindex tag, menu tag and etc.), frames, and no target links
(X)HTML Transitional: you can use deprecated tags(basefont tag, center tag, font tag, strike tag or s tag, u tag, applet tag, dir tag, isindex tag, menu tag and etc.), targeted links, but no frames(if you have frames, you have to use the Frameset doctype, but iframes don't count)
(X)HTML Frameset: it allows every thing Transitional does, but you can have frames(iframes don't count)

HTML 4.01:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd";>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">

XHTML 1.0:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">