There are some jokes almost anyone can get, and then there are ones that can only be understood by people with certain knowledge. Jokes about computer programming that can only be understood by technicians, jokes about a film that only make sense if you listened to the director's commentary, jokes about a country's political system or culture that only citizens of that place could ever laugh at. If you are in on the joke, these can be hilarious; if you aren't, then they get irritating fast.
From a sociological point of view, in-jokes are used by groups to identify those who are Not Us — anyone who's not a member of the group is not likely to get the joke.
If these are frequently placed in a show or book, then it can become a case of Viewers Are Geniuses, or if they are language related, Bilingual Bonus. Fandoms are often teeming with them, so seeing a certain one everywhere may be a result of Fan Myopia. In some cases, may even lead to Memetic Mutation. If the group is big enough, or is generally a Fountain of Memes, it is easily confused with Memetic Mutation, and often is on this very wiki. However, as long as the particular in-joke stays confined to the group itself it is not an entire internet meme, although attempts may be made to force it from time to time, some successful. If the creator acknowledges the joke or meme in the work itself, it becomes a Fandom Nod or an Ascended Meme.
- Bilingual Bonus: Something is said or written in a foreign language.
- Call-Back: A reference is made to the events of an earlier installment.
- Call-Forward: A prequel or flashback alludes to events that the audience knows will eventually happen.
- Continuity Nod: The events of an earlier episode are acknowledged.
- Creator In-Joke
- Development Gag: The work references a concept from when it was still in development.
- Discontinuity Nod: The characters make a disparaging reference to a reviled part of the franchise.
- Genius Bonus: A joke that you'll only get if you're very knowledgeable about certain obscure subjects.
- Historical In-Joke: A joke alluding to a historical event.
- Inspiration Nod
- Meta Casting
- Mythology Gag: The current adaptation makes a reference to one or more of the previous continuities.
- Production Throwback
- Remake Cameo: A remake or reboot features a cameo by an actor who had a role in the original version.
- Running Gag: A joke that repeats throughout the series or even a single installment.
- Shout-Out: A work references another work.