OAuth - simple as this diagram!

OAuth (Open Authentication) is an open standard for authorization. It allows users to share their private resources (e.g. photos, videos, contact lists) stored on one site with another site, desktop or mobile application without having to hand out their credentials, typically username and password.(http://en.wikipedia.org/wiki/OAuth)

OAuth sequence diagram
from
http://www.sitepen.com/blog/2009/02/19/introducing-oauth-in-dojox/