Memes (rhymes with dreams) are similar to a computer virus, in the way they spread quickly over a network, or group of computers. The difference is that they are a sort of virus of the human mind. An idea, or thought that spreads quickly, or one that occupies your thoughts that is tough to remove.
Begining a chant at a ball game or rally. A new dance craze. A song that you hear that you just can't get out of your head.