If you have an account (with some private resources) in a website, you can log in with username/password couple.
If an application would like to get some private resources, and if you don't want to give them your username/password, use OAuth.
But if you want to log in into multiple websites with a unique account, use OpenID.
(Some websites use OAuth like OpenID, and OpenID can be use like OAuth if you have some private stuff in your OpenID account)
