Principal Consultant, SiliconEdge
Depending on who you listen to, it's either the fastest growing "new" field of marketing or one of the most over inflated of buzz words.
My take as a "classically-trained marketer" is this:
It ain't nothing new.
It's just a form or better yet an updated framework for direct response advertising. That being said, I do think that term can be / could be useful as a short hand term for:
"A super-focused, super-aggressive form of Direct Response Advertising where your ass, your paycheck and your company's survival is on the line everyday, and whereby the marketing campaigns that you develop and run must be done so on a compressed time scale with a limited to non-existent budget."
Other than that, Growth Hacking can be said to be the same "stuff", different pile.