Authenticating user using a web service

Well, I have asked this question before but it seems people don`t understood me, my intension is to have a second login option like, user may choose to login with they facebook/myspace/etc id or the local id (my site) on my site, I want a situation whereby anyone can choose to register or use an existing social network id and password to login, so I am thinking of using facebook, but i don`t really understand something about facebook application, the application always, first go to facebook then redirect back to the site (callback url), I want all this process done under the hood. Can someone give me another idea? I will just love any simple idea that will help.