A meme
is basically an idea that takes on a life of its own and passes from person to person like a contageon. A good example would be the "All your base" thing from a few years back, or lolcatz. On b3ta memes consist of things like the Death Star, the Quo, Teh Fear and other altogether-too-popular bandwagons.

Google for the term "memetics" to learn more.

EDIT: here's a definition from Wikipedia.

A meme (pronounced /miːm/[1]) consists of any unit of cultural information, such as a practice or idea, that gets transmitted verbally or by repeated action from one mind to another. Examples include thoughts, ideas, theories, practices, habits, songs, dances and moods and terms such as race, culture, and ethnicity. Memes propagate themselves and can move through a "culture" in a manner similar to the behavior of a virus. As a unit of cultural evolution, a meme in some ways resembles a gene. Richard Dawkins, in his book, The Selfish Gene,[2] recounts how and why he coined the term meme to describe how one might extend Darwinian principles to explain the spread of ideas and cultural phenomena. He gave as examples tunes, catch-phrases, beliefs, clothing-fashions, and the technology of building arches.
( The Resident Loon Not a demographic. Do not measure., Wed 14 May 2008, 20:34, Reply)