Here is a joke that's supposed to be funny to nearly everyone. Maybe I'm a little outdated on my universal humor, but here it is. You may have heard it before:
A cop gets a 911 emergency phone call from a man-
Man: Help! I found him here! I think he's dead!
Cop: Slow down slow down! Check if he really is first!
(Too loud gunshots ring out over the phone)
Man: Yep, he's dead.
Hahahaha, funny funny, that's a small piece of simple humor. It is also an inside joke. In a way, I think ALL jokes are inside jokes to some extent, though some have a much larger audience. Consider the joke above that made you laugh uproariously. It's funny because we understand that:
1: The gunshots were probably from the man into the person who is possibly dead already
2: The officer probably didn't mean to make sure he was dead in the sense that he kill him again to make it certain.
Usually when we think "inside joke" we think something like a nerdfighter idea such as a specific John/Hank Green quote or "Pizza Face". A non nerd fighter wouldn't get that reference at all and wonder what pizzas had to do with faces besides going inside a part of them. (If you laughed at that it's because you're in on the joke that "part of the face" refers to the mouth, though that's not usually what it's called).
However, all jokes are inside jokes to people who like to think of things that are "out of the ordinary", in a certain way, are funny. Like the shooting of the uncertainly dead guy is humor. If that actually happened, the cop would probably have rushed over and arrested him for one reason or another. But, people who think in a joke context realize it's a fantasy world where the humor comes from the misunderstanding of the cops words by a dim witted man. A largely universal inside joke, but there are probably people who don't really "get" it, which is fine.
Another way to think of it is like this. When's the last time you heard a joke and didn't get it? When all of a sudden, you think it over again and *voila* NOW you understand and laugh, albeit a little late (which can cause a bit of laughter from the people who understood the joke before you). While you were still stewing over how it was humorous, you were on the outside of the joke. When, having adjusted your thoughts, you capture the humor in it, you are now part of this inside joke which is an inside joke to anyone else with a similar thought pattern that you just went through.
Jokes are inside jokes to those who get them. And that's all humor.