DTD

Definition
According to the W3C:


 * DTD
 * A DTD, or document type definition, is a collection of XML markup declarations that, as a collection, defines the legal structure, elements, and attributes that are available for use in a document that complies to the DTD.

Usage
Using XHTML Strict as an example, one would insert the following at the head of the document. This would tell user-agents what sort of document this was and in what terms to interpret it.

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

Note
Keep in mind that DTDs have nothing to do with browsers and XHTML as such. XHTML is one small family of innumerable DTDs and browsers are just one interpretative application. A spreadsheet file format may be in XML and a spreadsheet application may render that data and so on.