Entertaiment is a form of art which can be fun for people to consume. It requires consumer activity, can be objectively defined, has external stimulus, offers pleasure, needs an audience and is passivity based. It can stimulate the brain to realease seratonin, dopamine and other chemicals which give a good feeling. Often entertainment hit the points in life which are very deep for the human mind, like social backstabbing and murders, because the humans were evolved to deeply react on them.
Moreover, the activity of entertainment can make you happy and also relieve stress, as it can be a distraction from the daily routine. It releases endorphins which are the hormones that reduce pain and stress in the body.