A list apart has a nice write-up of the forthcoming HTML 5 standard.
If you are like most designers, you probably don’t write all your markup by hand. But until the tools you use catch up to the new elements in (X)HTML 5, you will be doing some markup by hand while you learn. There’s been a bit of confusion (and controversy!) about the relationship between HTML 5, XHTML 1.0/1.1, and XHTML 5. Let’s clear that up right now.
HTML 4.0 (the markup language we all know and love) is based on a “rulebook” called SGML. In the SGML rulebook, element names are not case sensitive, you can have elements with optional closing tags (like <p>), and you can have attribute values without quotation marks. XHTML 1.0 and 1.1 are based on a rulebook called XML. In the XML rulebook, element and attribute names are case sensitive, every opening tag must have a closing tag, and attribute values must be quoted.
HTML 5 defines a markup language that isn’t based on either rulebook, but that can be written in either “HTML form” (or serialization, as the spec calls it) or “XHTML form”.
Excellent – so now we all have an easier standard to work with that is more flexible, less demanding, and .. oh yeah: is only supported in the absolute newest of browsers.