A man walks into a talent agent's office, and says, "We're a family act, and we'd like you to represent us."
The agent says, "Sorry, I don't represent family acts. They're a little too old-fashioned."
The man says, "But, this is really special."
The agent says, "Okay, well what's the act?"
He replies, "Well, my wife and I come out on stage and she begins to sing the Star Spangled Banner while I take her. After a minute of this, my kids come out and begin to do the same."
The agent looks uncomfortable, but the man continues, "Just when my daughter hits the highest note in the song, my son and I switch partners."
The man smiles fondly as he recalls, "This is the best part: our dog then comes out on the stage, and he's trained to lick each one. He just goes right down the line, looking as happy as can be! We all get up and take a bow."
He looks at the agent and says, "Well, that's the act. What do you think?"
The agent just sits in silence for a long time. Finally, he manages, "That's a hell of an act. What do you call yourselves?"
"The Aristocrats!"
It's Chevy Chase's version. I editted it heavily...fill in the blanks yourself.
The object of the joke is to make the act as repulsive as possible. Some versions go on for hours.