Directly from MetaFilter:
South Park does the “Aristocrats” joke. (WARNING! Windows Media file, very very not safe for work.) “The Aristocrats” is a long-lived comedians’ in-joke–or, rather, an extraordinarily filthy joke that’s not really a joke. (Gilbert Gottfried knocked ’em dead with it shortly after 9/11.) Now it’s going public (sort of): Penn Jillette and Paul Provenza are making a movie featuring over 100 comedians telling their own versions. The South Park version linked above is “not even in the top 5 for dirtiest.” Yikes!
Comments: The first link video is safe for work, its just the south park gang, the audio is definitely headphone only material.
I guess the whole thing is just a comedian’s competition on who can come up with the filthiest/funniest version of the bit between the setup and the punchline. The punchline not being funny at all.
I find that I use a humor compatibility test using certain things from movies and TV shows. For example, I think the funniest part of the clip is Cartman doing the “hold on Kyle’s” in the middle of the story.