Microsoft has uploaded a video on YouTube that explains how they built the UI for windows

Here is another video showing the OS in action

Microsoft has also released a press release that highlights some of the new features. Here are some of them

Fast launching of apps from a tile-based Start screen, which replaces the Windows Start menu with a customizable, scalable full-screen view of apps.

Live tiles with notifications, showing always up-to-date information from your apps.

Fluid, natural switching between running apps.

Convenient ability to snap and resize an app to the side of the screen, so you can really multitask using the capabilities of Windows.

Web-connected and Web-powered apps built using HTML5 and JavaScript that have access to the full power of the PC.

Fully touch-optimized browsing, with all the power of hardware-accelerated Internet Explorer 10.

You can find the press release here

It is interesting that you will use HTML5 and JavaScript to build apps. Steven Sinofsky also mentioned that SilverLight will run in the browsr, he did not mention anything about out of browser apps with SilverLight.

If you want to learn about how to build all this new stuff then check out the Build Conference in Anaheim between September 13th and 16th.

BUILD is a new event that shows modern hardware and software developers how to take advantage of the future of Windows. Learn how to work with the all new touch-centric user experience to create fast, fluid, and dynamic applications that leverage the power and flexibility of the core of Windows, used by more than a billion people around the world.

More info here:

Finally you can read about what Steven Sinofsky showed at the D conference here: Microsoft Windows President Steven Sinofsky Live at D9