Basically, the DOCTYPE describes the HTML that will be used in your page.
Browsers also use the DOCTYPE to determine how to render a page. Not including a DOCTYPE or including an incorrect one can trigger quirks mode.
The kicker here is, that quirks mode in Internet Explorer is quite different from quirks mode in Firefox (and other browsers); meaning that you'll have a much harder job, trying to ensure your page renders consistently with all browsers if the quirks mode is triggered, than you will if it is rendered in standards mode.
Wikipedia has a more in-depth summary of the differences in rendering when using various DOCTYPEs. XHTML is enabled by certain DOCTYPEs, and there is quite a bit of debate about the use of XHTML which is covered well in XHTML?—?myths and reality.
There are subtle differences between different "standards compliant" rendering DOCTYPEs, such as the HTML5 DOCTYPE (<!DOCTYPE html>, prior to HTML5, only known as the "skinny doctype" which does not trigger standardized rendering in older browsers) and other DOCTYPEs such as this one for HTML 4.01 transitional:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
