DTD stands for Document Type Definition. In XHTML, the page author must specify which DTD the document supports. You can think of the DTD as being the list of XHTML language rules that must be abided by for the document to be considered valid.
XHTML 1.0 has three possible DTD’s: strict, transitional, and frameset. The strict DTD is, as its name implies, the strictest definition of XHTML. Several tags have been removed from the language (like CENTER), and even some attributes of other tags have been removed too (like the ALIGN attribute of H1). Transitional DTD supports most of the standard HTML 4 tags and attributes, while the frameset DTD is only for documents which hold frameset definitions.
The three DTD’s are:
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"DTD/xhtml1-strict.dtd">
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"DTD/xhtml1-transitional.dtd">
PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"DTD/xhtml1-frameset.dtd">
The DOCTYPE command has to be the second line of the XHTML document, after the <_xml_> command..