HTML was developed by the W3C until 2004, when members of the HTML working group grew disturbed with the direction the W3C was going with HTML. They felt that the W3C was not paying enough attention to the real-world development needs of the language and focusing too much on XML and XHTML. So they formed a new group called WHATWG (Web Hypertext Application Technology Working Group) devoted to evolving the Web. They started by working on a new specification of HTML - HTML5.
HTML5 is a new version of HTML 4.01 and XHTML 1.0 focusing on the needs of Web application developers as well as evolving HTML and addressing issues found in the current specifications.
You Can Use HTML5 Right Now
While the HTML5 specification (also on the W3C as a Working Draft) is not finalized yet, you can use it with any browser that supports it or any part of it. For example, many Web browsers currently support the [a href='http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html]canvas element. This element is used to draw graphics with scripting. It is currently supported by Safari, Firefox, Opera, and IE 8.
Why Should You Learn HTML5
HTML5 is the newest specification for HTML, and many browsers are going to start supporting it in the future. One nice thing about HTML5 is that it attempts to stay backwards compatible. So if you don't want to learn it just yet, you don't need to.
If you build Web applications you will eventually want to learn HTML5. There are a lot of new attributes and tags built just for Web applications. For instance, there are a number of new event handlers for drag and drop:
And many more.
At this point in time, there is not a lot of support for HTML5, but that support is growing all the time. By staying abreast of the changes you'll be prepared when they become widely available for use.