Over the last few months I’ve been working on discovering the functionality of the OAuth standard. OAuth (Open Authorization) 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 without having to hand out their credentials, typically a username and password.
So lets assume some third-party website like Dropbox wants to import your photos from Facebook. The way this would work is Dropbox would first redirect you to Facebook.com. If you had been logged into Facebook already it would ask you to allow the Dropbox website access to your Facebook information, in this case your photos. If you had not been logged in it would ask you to log into Facebook as usual and then would continue as stated above. Once you had allowed Dropbox access, Facebook would redirect you back to the Dropbox page and the download of your photos would begin.