Bad Wolf

A clip on YouTube where Bad Wolf is explained (from a Doctor Who Confidential?)

David Tennant: When you knew that you were bringing Billie back, Rose back, did you know that Bad Wolf would be...
Russell Davies: Its part and parcel of Rose, really, as a warning, as a message, as a sign that goes with her. Its a warning of trouble to come...the end of the universe. Because if those words are starting to reappear again then things are as bad as they possibly can be."

David Tennant: ...What did Rose do when she infected the world with Bad Wolf?
Russell Davies: It was a message that she created for herself. Rose absorbed the time vortex and in order to put herself in a position where she could absorb the time vortex, when she'd absorbed the time vortex she sent a message back through time that was a warning, a signal, and also an act of faith... its important... two word?...nursery rhyme words...resonate. Sorta saying you must be here at this time to become the Bad Wolf. She calls herself the Bad Wolf; that is Rose with all the power of the time vortex.

Billie Piper: Its a warning sign for Rose but its also a warning sign for the Doctor. So as soon as he realizes Bad Wolf, or Donna tells him, that's his way of knowing that Rose is on the hunt and that they may meeet again.

Russell Davies: Rose was trapped in a parallel universe. The walls are closed. Opening those walls was gonna mean the deaths of all universes...?...If messages are bleeding through, if she's...crossing between?...universes everything is in danger. Not just this universe but every universe.
David Tennant: She's using Bad Wolf to let the Doctor know ahead of time
Russell Davies: Yes.