On the web:
oAuth acts like a proxy for your credentials, allowing you to select where and what information is shared between applications. For example, when you “login with Facebook” you are saying that you authorize Facebook to share a subset of your information with a new application.
oAuth is a token based secure authorization protocol that is often used to connect to an application’s API. The advantage of oAuth is that access (the token) is usually only granted on a temporary basis and can be revoked easily.