Because Microsoft is a major player in the standards body that's trying to finalize HTML5, it put its version of the language in the new browser. Websites written with HTML5 can work across platforms, on different kinds of operating systems -- Mac, PC and mobile browsers. So developers using HTML5 need to design only one version of their site, which of course drives down production costs.
Microsoft has scheduled an event here at SXSW to launch IE9 Monday, and the browser will become available for download Monday night at 11 p.m. Central Time. If you simply can't wait to see it, you can download the "release candidate" here.