HTML

• Hypertext Markup Language, a standardized system for tagging text files to achieve font, color, graphic, and hyperlink effects on World Wide Web pages
• HTML is the code that tells browsers how to display a page on your screen. This code, sometimes called "markup", uses "tags" that instruct a browser to make text a certain size or style, display an image, or link to another page or web site.

XHTML

• Extensible Hypertext Markup Language, an HTML system for tagging text files to achieve font, color, graphic, and hyperlink effects on World Wide Web pages, incorporating user-defined elements.
• (Extensible Hypertext Markup Language) is a family of XML markup languages that mirror or extend versions of the widely used Hypertext Markup Language (HTML), the language in which web pages are written.