Harassment is defined as
to trouble and annoy repeatedly and which would include bullying.
Harassment comes in many forms - physical, emotional, sexual. You might think that your mother going on and on about you tidying your room could be called harassment, and you might tell her that, but you would hardly want to take her to court over it, and anyhow she might accuse you of harassment over a whole range of things like asking for pocket money all the time!
So where does normal social behaviour end and 'harassment' start? It is a fine line, but we usually realise when that line has been well overstepped, like stalking; or when someone goes on and on trying to make someone take an illegal drug when they don't want to. Or someone is deliberately trying to make your life hell and miserable over a long period of time.