What Is OAuth? How Those Facebook, Twitter, and Google Sign-in Buttons Work

If you’ve ever used a “Sign In With Facebook” button, or given a third-party app access to your Twitter account, you’ve used OAuth. It’s also used by Google, Microsoft, and LinkedIn, as well as many other account providers. Essentially, OAuth allows you to grant a website access to some information about your account without giving it your actual account password.
by Chris Hoffman via How-To Geek