An inside joke is a joke that originates from a shared experience that sets up an opportunity for humor. You and your friends may see a homeless man screaming phrases at a trashcan; later, you repeat those phrases and your friends double over in laughter.

An inside joke may also arise from a shared cultural experience. For example, if someone posts to AskReddit: "How long does it take to learn how to draw?" and someone answers "42. Hell, it's gotta be the answer to something, why not this?"

Although that's a lame pseudo-joke, the "42" concept is an excellent example. It cannot propagate on its own. It is extremely unlikely that someone who hasn't read HHG2G will replicate this joke, even if the absurdity of saying 42 elicited a chuckle.

A meme, on the other hand, replicates on its own. "Who was phone?" has multiplied quickly despite very few people looking up the source. Use of "FAIL" and "WIN" is ubiquitous, but almost none of the people participating are aware of the Scientology connection.

This doesn't take blind copying. The idea behind the meme is deeper than syntax. On slashdot in 2004, someone tried to meme an article about how in Korea, only old people use e-mail. It was obviously the soviet russia meme in disguise, or giving birth. The deeper concept is that a consistent transformation can be applied to an ordinary English sentence to yield a potentially humorous sentence about life in a foreign country. If the Korea fork had been successful - it wasn't - then future generations of netheads might have gone on blithely making old korean jokes without any knowledge of the original soviet joke.

Again, an inside joke just can't undergo that kind of mutation. The inside joke requires some share in the original experience.

TL;DR "Memes" and "inside jokes" are subsets of "cultural references" with a non-null intersecting set.