Most people like to put their personal stamp on their PC with their own desktop wallpaper and settings that have been tweaked just the way they like them. But all that is lost when they log onto a shared PC on which they don't have an account set up. All that changes with Windows 8, which allows users to enjoy their own profile, settings and Metro style apps on any Windows 8 PC by logging in with their Windows Live ID.
In Windows 8, users will be able to associate the most commonly used Windows settings with their Live ID user account so the settings will be available when logging into any Windows 8 PC. As it stores all the information associated with your Windows Live ID in the cloud, when you log into another Windows 8 PC for the first time it will download the relevant information in the background and apply the settings as it comes in so you'll still be able to use the PC and not have to wait until all the data is downloaded before getting to work.
Additionally, users' sign-in credentials will be saved for various apps and websites so, after logging in to the PC, you won't have to enter your log in details again for services and applications that also use Windows Live ID, such as Hotmail. Metro style apps will also stay in sync across multiple PCs and you'll be able to see which apps you've purchased and choose which ones you want to have on separate Windows 8 PCs.
Metro style apps that have been built to tell Windows their state will also resume right where you left off when switching between PCs - remembering which point in a video, which page in a book or which level in a game you were up to, for example.
Logging into a Windows 8 PC with your Windows Live ID will mean you'll also be automatically signed into apps and services that use Windows Live ID for authentication. Additionally, you'll be able to store separate app and web site credentials that will sync to each Windows 8 PC that you've verified yourself with so you'll just have to confirm your sign-in rather than entering your name and password each time.
Microsoft has broken the various settings that can be synced into three categories: Windows settings, App settings and data, and credentials. Microsoft says the size of the data is small and it only enforces some limits on a per setting basis - for example, the desktop wallpaper image is limited to 2 MB and if it exceeds this limit the image is compressed and cropped to 1920 x 1200. Synching can also be turned off and on by the type of setting, which is broken into the following groups: Personalize, Themes, Ease of access, Language preferences, Apps, Web browser, Other stuff and Some passwords.
Signing in with a Windows Live ID isn't compulsory but rather an option. When creating a Windows account, you'll be able to choose whether it will be associated with a Windows Live ID, or will just be a local account that works in the same way as current Windows 7 accounts. If you choose the latter option, you'll be able to change your mind and link the local account with a Windows Live ID at a later date.
Source: Building Windows 8 Blog