The HTML Doctype is used to tell a browser which version of HTML your page is written in. Thus it is very important that it is included on each HTML page. With HTML5, the doctype will be changed to be significantly shorter and easier to remember.
Here is the HTML5 Doctype:
You can see how much this differs from the common HTML 4.01 doctype:
<!DOCTYPE HTML PUBLIC “-//W3C//DTD HTML 4.01 Transitional//EN” “http://www.w3.org/TR/html4/loose.dtd”>
For more info on Doctypes see W3schools