Bullying is a form of destructive behaviour, aimed at a person or group, usually by one person, whose aim is to undermine and abuse the target, to make him or her look and feel inferior to others.
It is done for various reasons, not all known or readily observable to us. It’s usually done subtly, as the perpetrator realises that if others see him or her behaving in a bullying way, it will have the reverse effect of making him or her (perpetrator) look bad.
The intention is to focus negative attention on the target individual. It is a type of passive aggression, in that is seldom involves physical acts of violence, but instead relies on indirect acts which will ultimately hurt the target, psychologically and emotionally.