The main purpose of Joke Programs are to annoy or entertain the user. Some can be made with the intent to scare the user into thinking their machine is damaged or broken, but in truth, their machine is fine. Joke programs can sometimes be detected by Antivirus programs because they may interfere with the normal work of a computer.
They do not infect files, nor do they actually cause damage that is not told to the user beforehand. None are self-replicating, although they could be conceivably be infected with other malware. Some joke programs are damaging, such as WinFastener and Lose/Lose.