In general, aggression is a form of animal behaviour characterised by an assault or attack on one animal by another This can take the form of conflict between members of different species for the purpose of obtaining food or defence, or attacks directed towards members of the same species, for example, in the way that goats butt their heads together.
In humans, the term aggression is a general term used for a wide variety of acts including attack or hostility towards one another, and this can be caused by such factors as fear or frustration, a desire to produce fear or flight in others and a tendency to invoke one's ideas or interests on others.
In humans learned experience is important in determining the level of aggressive behaviour, and generally the trading of insults or the presence of weapons have all been learned from various sources.
The obtaining of rewards by children, such as toys, attention and sweets, as a result of aggressive behaviour, is also likely to reinforce such behaviour. Children also learn aggression by observing others, by having role models and by the influence of the mass media.
In general, the kind of aggression that most people tend to think of when the term is used is aggression evoked by frustration or the thwarting of one's goals. It is also typified by a display of the will to power and the desire to control others.