In-Jokes
An "in-joke" is one which three or fewer people understand, and forms the bedrock of Encyclopedia Dramatica.
For example, an ordinary joke might be as follows;
Q. Why did the chicken cross the road?
A. To get to the other side
whereas an in-joke might be;
Q. Why did the chicken cross the road?
A. Because of a point mutation in the gene coding for its sinal magnetite receptor.
Other types of joke
In-jokes can be distinguished from unfunny jokes (see GET OUT) or surreal jokes ...
Q. What's black and white and red all over?
A. A gooseberry in a lift.
Q. A horse walks into a bar what does the bartender say? A. Why the long face.
... in that at least other other person, possibly in a covert organisation or a cult, gets the joke and then laughs at it.
Other examples of In-jokes
(1) PC users
Q. Why did the penguin cross the road?
A. Because it was the Lunix corporate logo.
(2) Ku Klux Klan
Q. Why did the black bird cross the road?
A. Because it saw a white woman.