What are the levels of granularity you can implement, to make sure the right people can access your applications,  but non-authorized users are kept out? Learn how to configure your apps to be accessible to anyone, or to restrict access to Google Account users, to users at a specific Google Apps domain, or to anyone who has an account with an OpenID provider.
Once your users have logged in, how can you personalize the experience for them? This presentation discusses how to use the Users API to get basic information about the user, and gives code samples to get you on your way to  providing a personalized experience for each user.
Learn how to use OAuth to allow a user to grant limited permission to a third party (be it another user or a web application) to access a web application on their behalf.