A clean joke is a joke that avoids the use of dirty humor, specifically the use of curse words and sexual content, in order to keep the joke inoffensive. Some American comedians specialize in clean or family humor as part or the whole of their act.
Clean humor is the use of jokes and routines that lack offensive content, making them fine for children to experience. Comedian Bob Newhart is notable as an example of a comedian who tells jokes without making them dirty; instead, he takes the idea of a straight man to its extreme by performing acts where he is reacting to some imaginary insane situation. He went on to do acting and other works partly because of the popularity of his unique brand of comedy.
Some examples of clean jokes can be found on LaughFactory.com, with reviews of the jokes being given ratings in the form of up- or down-votes. The jokes found there range from simple wordplay, to jokes about religion or evolution and strange shifts in perspective.
Many clean jokes rely on wordplay, taking expected answers and twisting them. Dirty jokes often rely on some measure of shock in the audience to get a laugh, in contrast.