From the introduction:
The release of Windows Phone 8 marks a significant milestone in the evolution of Microsoft's operating system for mobile phones. For users this new release means expanded hardware access, support for high-end gaming, and better OS integration - all of which will soon be seen on the Nokia Lumia 920 and Nokia Lumia 820 phones.
For developers the changes are no less significant, bringing many new APIs. These APIs simplify the coding of features you may already be adding to your Windows Phone apps, but also offers many new ones too. This article will introduce you to the most significant API additions and changes.
Many of the new features of Windows Phone 8 are highlighted here, including native C++ coding, improved DirectX 3D graphic support, voice recognition, and how developers can work with the new and improved lock screen.
You can get started at the Nokia Developer wiki page here.