Every HTML(Web Page) document must begin with a DTD before the tag. it is an instruction to the web browser about what version of HTML the page is written in. Here are some common example Type of DOCTYPE and DOCTYPE uses.

HTML5

<!doctype html>

HTML5 is the most current standard, and it is the DTD you should be using right now. It adds a lot of great new features like sectioning elements, new semantic elements, new form types, new multimedia elements, and a lot of great APIs to extend your web applications.

HTML 4.01 Strict

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

HTML 4.01 Strict removes all the deprecated tags from the specification. This DTD contains all HTML elements and attributes. Framesets are not allowed.

HTML 4.01 Transitional

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

This DTD is very loose and does not provide as much consistency across browsers. That means that pages you build with it are going to be more difficult to maintain and more likely to look different in different browsers.

This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements. Framesets are not allowed.

HTML 4.01 Frameset

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

This document type declaration is equal to HTML 4.01 Transitional, but allows the use of frameset content.

XHTML 1.0 Strict

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

Framesets are not allowed. The markup must also be written as well-formed XML.

XHTML 1.0 Transitional

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

This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML.

XHTML 1.0 Frameset

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

You can use this when you want to use HTML Frames to partition the browser window into two or more frames.

XHTML 1.1

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

This DTD is equal to XHTML 1.0 Strict, but allows you to add modules (for example to provide ruby support for East-Asian languages).