Entertaiment is a term that covers a very wide range of entertainment. It can be as simple as a private movie viewing for two or as complex as an event that is broadcast to thousands. Some forms of entertainment are as old as human civilization itself, while others have developed as new technologies allow for the expansion and refinement of the art form.
Entertainment often hits on themes that the brain was evolved to deeply react to, like social backstabbing and murders. It also stimulates the brain to realease seratonin and other chemicals.