This is a big question, and answer to this makes anger management easy, i.e. if its known that what actually anger is and why do we get angry, we can identify the causes and can stop the effects. In this regard I found a agood introduction of anger here on Buzzle.com. Below is the excerpt:

Anger is a deluded mind that has its focus on an animated or unanimated object, feels it to be very unattractive, and exaggerates its bad qualities and wishes to cause harm. It is the most common destructive delusion, and afflicts our mind almost everyday.

Anger is a response to feelings of unhappiness, which in turn arise whenever we meet with unpleasant circumstances. Whenever we are prevented from fulfilling our wishes, or forced into a situation we dislike, or rather we are put into a situation we want to avoid- our mind reacts immediately feeling unhappy and discomforted. This uncomfortable feeling easily turns into anger.

Another important reason for our anger is that in our day to day life we face many situations which we do not like. Everyday we face situations like stubbing our toes, arguments with our peers, and even drastic situation like hearing bad news like our house being burnt down and even a death in our family. The most common reaction of a normal person is to feel unhappy and angry. We are not in total control of ourselves.