XHTML

HTML (HyperText Markup Language)

An application of SGML for marking up the logical structure of web documents
CSS (Cascading Style Sheets) should be used to control the physical layout of online documents.
General info about HTML
HTML 4.01 (W3C recommendation)

XHTML (Extensible HyperText Markup Language)

A reformulation of HTML as an application of XML
General info about XHTML
XHTML 1.0 Second Edition (W3C recommendation)
Authoring Techniques for XHTML & HTML Internationalization: Characters & Encodings 1.0 (W3C working draft)
Authoring Techniques for XHTML & HTML Internationalization: Handling Bidirectional Text 1.0 (W3C working draft)
XHTML Modularization 1.1 (W3C working draft)
XHTML Basic 1.1 (W3C working draft)
XHTML-Print (W3C recommendation)
XHTML Media Types (W3C note)
XHTML 1.1 Second Edition (W3C working draft)
XHTML 2.0 (W3C working draft)
Microformats

Major Differences of XHTML from HTML

Documents must be well-formed.
Element and attribute names must be in lower case.
For non-empty elements, end tags are required.
Attribute values must always be quoted.
Empty elements must either have an end tag or the start tag must end with />.

Major Differences of XHTML 1.1 from XHTML 1.0 Strict

On every element, the lang attribute has been removed in favor of the xml:lang attribute.
On the a and map elements, the name attribute has been removed in favor of the id attribute.
The "ruby" collection of elements has been added.

Recommended Offline Reading

Musciano, C. & Kennedy, B. 2006⁶. HTML & XHTML: The Definitive Guide. Sebastopol, CA: O'Reilly.