Windows Live ID (Formerly known as Passport) is a single sign-on service offered by Microsoft. Setting up a Windows Live ID allows you to access several different applications with the same credentials. Some applications that use your Windows Live ID to authenticate you include Hotmail, Windows Live Messenger, and MSN. Xbox Live also uses your Windows Live ID! If there were no Windows Live ID service in place, you would need a separate ID and Password to access each of these services.