- OpenID is (mainly) for identification/authentication, so that
stackoverflow.com
knows that I ownchris.boyle.name
(or wherever) and therefore that I am probably the same person who ownedchris.boyle.name
yesterday and earned some reputation points. - OAuth is designed for authorization to take actions on your behalf, so that
stackoverflow.com
(or wherever) can ask permission to, say, Tweet on your behalf automatically, without knowing your Twitter password.