In terms of psychology, at least, aggression is defined as any act that is meant to bring harm to another person--to make them suffer or to in some way damage them. This means that aggression can take many forms.
For example, there can be direct physical aggression. Punching someone is meant to hurt them and is therefore aggression. Stealing from someone's home could be aggressive as the loss of valuable possessions would cause emotional pain to the person. Finally, words can be aggressive. We know that words have the power to hurt.
Additionally, the link below give detailed discussions of aggression and the causes of aggression.