Let's first begin by defining anger. Anger can be defined as one of the normal spectra of human emotions like joy, sadness and regret. Who isn't angry after they just discovered they overpaid for a health club membership, car or new bike? Anger is apart of the human condition. Rage, on the other hand, is an extreme manifestation of normal anger. People who are rage-full may actually have an organic disorder like substance abuse or underlying personality disorder. Separation of rage and anger depends on the frequency and intensity of the emotion. For example, if a person develops anger which lasts for days, weeks or months, then it is probably rage and no longer anger.
People with bipolar disorder, major depression and anxiety are prone to manifest rage. However, people who have substance abuse disorders can frequently demonstrate rage or be enraged. This is because illicit substances like alcohol can disinhibit a person's thinking processes and emotions. This is a very common source of rage.
On the other hand, a person with a personality flaw can demonstrate daily rage which can be traced years in duration. These are the people who are angry or pissed off at any or everything and never quite understand why. These are individuals who should seek psychotherapy or counseling in order to get to the source of their rage. Such rages can impair them occupationally, socially and personally.
Normal anger is apart of normal human emotion and is not in and of itself a sign of mental disorder.