Facebook provides some documentation on the parameters of oauth
login.
Login Dialog OAuth 2
Parameters are:
https://graph.facebook.com/oauth/authorize is also to logging in the person -- Like authenticating a person and to take permission from person whether to access the requested permissions by app.
oauth/authorize is graph api call. I think major difference may be when you want to build the login flow manually you should use /oauth/authorize.. else if you are using javascript/Apps api provided by facbook it uses /dialog/oauth. Apps normally need to confirm that the response from the Login dialog was made from the same person who started it. If you're using Facebook's JavaScript SDK it automatically performs these checks so nothing is required, assuming that you're only making calls from the browser. More over we can make graph api calls secure by applying appsecret_proof.