From http://www.w3schools.com (Copyright Refsnes Data)

XHTML DTD

The most common DTD is XHTML Transitional.

<!DOCTYPE> Is Mandatory

An XHTML document consists of three main parts:

The basic document structure is:

<!DOCTYPE ...>
<html>
<head>
<title>... </title>
</head>
<body> ... </body>
</html>

Note: The DOCTYPE declaration is always the first line in an XHTML document!

This is a simple (minimal) XHTML document:

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

<html>
<head>
<title>simple document</title>
</head>
<body>
<p>a simple paragraph</p>
</body>
</html>

The DOCTYPE declaration above defines the document type. The rest of the document looks like HTML.

A DTD specifies the syntax of a web page in SGML
DTDs are used by SGML applications, such as HTML, to specify rules for documents of a particular type, including a set of elements and entity declarations
An XHTML DTD describes in precise, computer-readable language, the allowed syntax of XHTML markup

There are three XHTML DTDs:

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

Use the strict DOCTYPE when you want really clean markup, free of presentational clutter. Use together with CSS.

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

Use the transitional DOCTYPE when you want to still use HTML's presentational features.

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

Use the frameset DOCTYPE when you want to use HTML Frames to split the web page into two or more frames.