From the site:
A Quick Overview
Windows Live ID is the identity and authentication system provided by Windows Live. Currently, more than 380 million users have credentials that work with Windows Live ID. And now, Microsoft offers two software development kits (SDKs) that make it possible for developers like you to use this powerful service.
By creating Web sites and applications that combine user authentication with other Windows Live services, you can offer your users new and unique online experiences that include the following:
Windows Live ID : Get Started with Windows Live ID Now