It's slightly simplistic to say that negative thinking is simply the opposite of positive thinking, but that essentially is the truth of it.
The term positive thinking out of the work of a guy called Norman Vincent Peale, and has come to refer to any type or thinking or emotional state that is considered good or healthy. The term negative thinking has really been coined as a consequence of the term positive thinking and in many ways does just mean the opposite. This however is a potentially dangerous term, not only because it is over simplistic, but because it is judicialal. Coming to think of your thinking in terms of being either positive or negative will lead you to continuously judge yourself, which will simply reinforce the labels that are already there, and simply increase the cycle of destructive thoughts and behaviors.
It is important to realize that the patterns of thought that are continuously referred to as negative, are in fact protective. That most people thought patters an behaviors at their root are designed to keep people safe and protect them from danger. This may well go back to cave men or not, but it is well known that mans most basic need is that of survival and our whole outlook on life stems from that. Protective thinking is our way of instinctively doing what we can to keep ourselves safe and out of danger. This may well result in some very warped and strange behavior patterns, but they're almost a bi-product of the process. Helping to understand where these thoughts came from and that the pattern of them is protective gives us a context that makes sense of them, and eliminates the whole positive and negative thinking debate