Microsoft announced that it is getting into the app business by previewing Windows 8 on Thursday. To build a “Metro style” app to be sold in the platform, you can use HTML5, CSS3, or  JavaScript.

Metro style apps features certain new controls including an “app bar” and “charms”. Here is an explanation about these new features from Microsoft’s developer page:

Outside of the app window, the app bar is the primary command interface for your app. Use the app bar to present navigation, commands, and tools to users. The app bar is hidden by default and appears when users swipe a finger from the top or bottom edge of the screen. It covers the content of the app and a user can dismiss it with an edge swipe, or by interacting with the app. The charms are a specific and consistent set of buttons in every app: search, share, connect, settings, and start. We believe these are core scenarios that every user wants to do in almost every app they use.

Follow this link for more details on how to make an app for Windows 8.