A joke is something told to make people laugh. There are many kinds of jokes; they can be questions with surprising answers or stories with unexpected endings. The humour of a joke comes from the surprise.
Jokes can be about anything. Jokes can be found anywhere: on the Internet, in books or heard from friends. Some jokes are about groups pf people and can hurt people's feelings, for example if they are racist or sexist.
Jokes could take the form of sarcasm or even try to poke fun at something to get a point across. People sometimes get hurt by jokes at them. If a joke is used to show a problem in someone's thought, it is normally called satire.