Anger is one of the basic emotions. It is an inherited response, and is common to all mammals and a number of other animals. It happens when we are threatened, offended, wronged, or denied something we really want or need.

Psychology of anger 
Anger is a normal emotion. It involves a strong, uncomfortable and emotional response to a provocation. There is a sharp distinction between anger and aggression (verbal or physical, direct or indirect). Both influence each other. Anger can start aggression or increase its probability or intensity. However, it is not necessary nor sufficient for aggression.

Anger is a bit like a pressure cooker: we can only apply pressure against our anger for a certain amount of time until it explodes.

People show anger to others by their face, what they do with their body, not trying to understand or help other people's problems, and sometimes acts of aggression or force in public (e.g. punching a wall). Animals and humans might try to scare- by making loud sounds, trying to make their bodies look bigger, by showing their teeth, or by staring.

When we face a challenge, our response may be anger or fear. It may be difficult to decide whether to face the challenge, or walk away. In animal behaviour terms, we face the choice of a fight or flight response.

Hormones and body changes 
Being angry changes the human body by making the heart beat faster, increasing blood pressure (the pressure made by the blood at right angles of the walls of blood vessels) and increasing amounts of the hormones adrenaline and noradrenaline (chemicals which send messages to parts of the body to make changes).

Being angry makes the levels of the hormones adrenaline and noradrenaline in the body go up, although this does not last for very long. Adrenaline quickly makes the body ready to act when there is important trouble by increasing the supply of oxygen and glucose (needed to make energy) to the brain and muscles, while slowing less important things the body is doing, like digesting food.

Noradrenaline is released during stress.  It goes to areas of the brain which control attention (how much you think or concentrate) and reactions. It is very important  when the body chooses "fight-or-flight" (attacking or running away). Then it increases heart rate, takes glucose out of storage so it can be used, and increases blood flow to muscles.

Symptoms 
Anger may be passive anger or aggressive anger.
These two types of anger have some characteristic symptoms:

Passive anger 
Passive anger can be expressed in various ways. Basically it means being angry, but not facing the issue at hand. 
 Walk away, turn one's back on a crisis, avoid conflict, not arguing back. Consequence: problem not solved, and tends to eat away at one's self-confidence.
 Psychological manipulation, such as provoking people to aggression and then patronizing them, provoking aggression but staying on the sidelines, emotional blackmail, false tearfulness, feigning illness, sabotaging relationships, using sexual provocation, using a third party to convey negative feelings, withholding money or resources.
 Indirect aggression, such as stockpiling resentments that are expressed behind people's backs, giving the silent treatment or under the breath mutterings, avoiding eye contact, putting people down, gossiping, anonymous complaints, poison pen letters, stealing, and conning.
 Obsessive behavior, such as needing to be inordinately clean and tidy, making a habit of constantly checking things, over-dieting or overeating, demanding that all jobs be done perfectly.
 Self-blame, such as apologizing too often, being overly self-critical, inviting criticism.

Aggressive anger
The symptoms of aggressive anger are:
 Bullying.
 Destructiveness, such as destroying objects as in vandalism, harming animals, child abuse, destroying a relationship, reckless driving, substance abuse.
 Hurtfulness, such as physical violence, including sexual abuse and rape, verbal abuse, biased or vulgar jokes, breaking a confidence, using foul language, ignoring people's feelings, willfully discriminating, blaming, punishing people for unwarranted deeds, labeling others.
 Selfishness, such as ignoring others' needs, not responding to requests for help, queue jumping.
 Threats, such as frightening people by saying how one could harm them, their property or their prospects, finger pointing, fist shaking, wearing clothes or symbols associated with violent behaviour, tailgating, road rage, slamming doors.
 Unjust blaming, such as accusing other people for one's own mistakes, blaming people for your own feelings, making general accusations.
 Unpredictability, such as explosive rages over minor frustrations, attacking indiscriminately, dispensing unjust punishment, inflicting harm on others for the sake of it, using alcohol and drugs, illogical arguments.

References

Other websites 
 Managing and controlling anger 
 Is Anger Worth It? Buddhist Video Teaching 

Emotions
Seven deadly sins
Basic English 850 words