Humour is defined by the Oxford English Dictionary as the quality of action, speech, or writing which excites amusement. Many theories exist about its nature and the social function it serves. For example, it is often pointed out that much humour contains an unexpected, often sudden, shift in perspective.
Throughout history, comedy has been used as a form of entertainment all over the world, from villages to courts. However, as with any form of art, the appreciation of different types of humour varies from person to person, and is affected by age, culture, and knowledge of the world. Children often respond to slapstick; but sarcasm requires a knowledge of social etiquette; verbal wit requires vocabulary; satire depends on the recognition of cultural norms; and parody depends on familiarity with literary tradition. What is certain, though, is that people of most ages and cultures respond to humour of one sort or another, smiling or laughing at something they consider to be funny.
The term derives from the medical theories of the ancient world, which held that human health and emotion was controlled by the proportions in which were mixed four fluids known as "humours" (Greek: χυμός, chymos, literally juice or sap; metaphorically, flavour).
Adapted from the Wikipedia article on Humour