A knock-knock joke is a type of joke told in the form of a conversation between a person knocking on a door and the person answering it. It always starts with the phrase "Knock, knock!" followed by the question "Who's there?" and ends with a punch line which usually has a pun or some other kind of word play.
The basic format is as follows:
Joke-Teller: "Knock Knock!"
Joke-Hearer: "Who's there?"
Teller: "[a word or phrase]"
Hearer: "[the word or phrase the Teller said] who?"
Teller: "[the punch line]"
Knock-knock jokes come from "do you know?" jokes in the 1900s and became very popular in the 1930s.