Windows 8 is the latest edition of the popular client operating system released by Microsoft. One of the most noted changes from previous versions is the new Modern UI (User Interface) which provides a full screen start menu and the ability to run Apps much like on a mobile device. There are two types of Windows 8 RT (believed to mean Real Time) which runs on ARM devices and is suited for Tablets and Mobile devices. Windows RT devices can only run applications delivered through the Windows Store due to the reduced instruction set.
The other version is Windows 8 and comes in two flavours: one for home use and one for enterprise / business use. This version can run both Windows Store apps and traditional desktop applications such as Adobe CS 6 or Visual Studio.
When building applications to run on the new platform it is important to understand the differences between Windows 8 Apps and traditional Windows Desktop applications. Windows 8 Apps will run on both RT and full installs of Windows 8, however these apps can only run full screen and can only be delivered through the Windows Store unless you are connected to an enterprise network in which case apps can be sideloaded where as traditional applications can be delivered through any supported medium.
There are extensive guides on building apps for Windows 8. There is the Windows 8 UX guidelines which determine the best practices for layouts and the SDK documentation which provides guidance on writing the supporting code to your UI. Both of these can be found in the Windows Dev Center.