I am very interested in animal behavior, and human's are animals, so much of it can apply. Most of what we see as negative emotion, is some form of arousal. Anger is arousal and as long as it's proportionate and appropriate to the situation - it's normal. In fact, it is even beneficial because it can spur one to some sort of action. Fear can often be confused with anger or come out as anger - one easy example, your kid does something incredibly stupid/dangerous and you're scared he could have killed himself, relieved that he is ok, and expressing it in anger.
As an emotion - a person or animal can be angry but still able to learn, to process information. Rage is what happens at the far extreme of the spectrum. With rage, there is no control, the body and brain are flooded with "flight or fight" hormones and learning/processing information is impossible at that moment.
I think anger is a healthy emotion, but rage is not.
It's like anxiety - a certain amount of anxiety is beneficial, it spurs us to better efforts, keeps us aware of what might be dangerous, but at it's extreme end, it can be crippling and disfunctional.
