Dr Who - What's Bad Wolf?

Can anyone help me out by explaining 'Bad Wolf' to me. I understand that it was a message to the doctor in season 1, and that it's reappeared in season 4, but who exactly is the message from? Rose or the Tardis? and what does it mean? I thought it was a link between the Dr and Rose, but after tonight's episode I guess it means the end of the universe? But why?