July 8th, 2010 7:21pm
Think of it like the cultural equivalent of a gene. A set of them define or are defined by a culture's or group's characteristics. In-jokes can be thought of as memes because they're specific to a certain group of people, the old Burma-Shave signs are a part of American culture from that time period, etc.
I don't quite subscribe to meme theory as Dawkins conceived it, but the word is really handy.
As for the things that have been quoted above, I direct you to Wikipedia:
http://en.wikipedia.org/wiki/Internet_meme