This article from A List apart is intended to provide a brief introduction to some of the major features as they are in the current HTML 5 draft.

From Abstract:

The web is constantly evolving. New and innovative websites are being created every day, pushing the boundaries of HTML in every direction. HTML 4 has been around for nearly a decade now, and publishers seeking new techniques to provide enhanced functionality are being held back by the constraints of the language and browsers.

To give authors more flexibility and interoperability, and enable more interactive and exciting websites and applications, HTML 5 introduces and enhances a wide range of features including form controls, APIs, multimedia, structure, and semantics.

Work on HTML 5, which commenced in 2004, is currently being carried out in a joint effort between the W3C HTML WG and the WHATWG. Many key players are participating in the W3C effort including representatives from the four major browser vendors: Apple, Mozilla, Opera, and Microsoft; and a range of other organisations and individuals with many diverse interests and expertise. Continue reading the article..