Entertaiment is the collection of all the ways that humans can amuse themselves and find enjoyment. This is a wide field, and entertainment can be found in many different forms, from a single performance to the entire infrastructure of a city devoted to providing public entertainment.
What may be considered entertainment by one group or individual can be regarded as work or cruelty to another. Entertaiment has the ability to cross over different media, which has led to a high degree of reuse and longevity of themes, images, and structures.
The reason for this is that entertainment often hits on the deep emotions our brains were shaped to react to. This includes themes of backstabbing, murder and other social situations that the human mind has evolved to understand as part of life on this planet.