A meme (pronounced /miːm/) comprises a unit or element of cultural ideas, symbols or practices; such units or elements transmit from one mind to another through speech, gestures, rituals, or other imitable phenomena.
In internet land, a meme is just another word to describe a giant in-joke.
I first came across the word when I discovered the Song Chart Meme on flickr. Here, a bunch of people with way too much time on their hands have created graphs and charts to describe their favourite songs. Like these. Can you guess which classic songs they depict?