Microsoft published the Preview release of Windows 8.1, users interested in testing this release can download it over here.
If you upgrade from Windows 8, you will be able to keep all Windows settings, your personal files, and most apps, but under some circumstances, you will not be able to uninstall the preview, making it impossible to upgrade to the final Windows 8.1 build without completely reinstalling the operating system. Full details can be found in the Windows 8.1 Preview FAQ.
An overview of some of the new features can be seen in the video clip below, it reveals the Start button, the changes to the Start screen, the new customization features, multitasking in Metro, and more. A more detailed overview of everything new can be found in the Windows 8.1 Product Guide (PDF).