401 "Unauthorized" really means unauthenticated. You need valid credentials for me to respond to this request.
403 "Forbidden" really means unauthorized. I understand your credentials, but sorry, you’re not allowed.
Source https://www.youtube.com/watch?feature=player_detailpage&v=FeSdFhsKGG0#t=394
It seems the video is no longer available in Youtube :(