Someone who likes to watch other people known as a voyeur. When this is continually done to a person it can have a bad effect.
Stalking in a way that makes someone feel unsafe is a crime in many countries. It is not just one event but an ongoing process that threatens or makes that person feel scared. Someone who does this is called a stalker. The stalker may do this because they want to be involved in that person's life, because they want to have power over that person's life, or for other sexual reasons. The stalker can make that person feel scared. They may feel they do not have the power to stop the stalker. Psychologists say stalking is a way of hurting someone's mind. Sometimes a stalker may go on to hurt that person's body by assaulting, raping or murdering that person.
Stalking can sometimes be done on the internet. This is called cyberstalking.