What is XHTML?

According to the W3C, XHTML is

a family of current and future document types and modules that reproduce, subset, and extend HTML, reformulated in XML. XHTML Family document types are all XML-based, and ultimately are designed to work in conjunction with XML-based user agents. XHTML is the successor of HTML, and a series of specifications has been developed for XHTML.

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• XHTML is an "XML-ized" version of HTML.
• XHTML is the bridge between HTML and XML.
• XHTML will (probably, eventually) replace HTML.
• XHTML 1.0 became a W3C Recommendation in January, 2000.