HTML5 is the next major version HTML, fixing a lot of issues with HTML4 and making the HTML language better suited for Web Applications than it is today. With the rise of Web applications, people started pushing HTML beyond its limits. Originally designed as a language to mark up documents, Web developers have since started using it for purposes including Web forums, photo sites and advanced mapping applications. By pushing HTML to its limits not only became it clear that it was not really suitable for applications, it also became evident that the HTML4 specification -- now over a decade old -- was not clear enough in details to foster interoperable implementations. HTML5 is an effort, started in 2004 by Opera, Apple, and Mozilla, to study contemporary HTML implementations and deployed content. Based on those studies, the HTML5 draft defines clear processing models for its features, e.g. for parsing HTML on the Web, and introduces various new features for Web applications so far only possible through plug-ins, e.g. displaying video or manipulating graphics.
Anne van Kesteren works for Opera Software in evolving the Web. As part of this activity, he's a member of a number of W3C Working Groups (WGs), including the WebApps and HTML WGs. He's also been a member of the Web Hypertext Application Technology Working Group (WHATWG), the group that founded HTML5 in 2004, since its inception.