Probably the most famous atheist on the planet, British biologist Richard Dawkins is somewhat less well known as the inventor of the word "meme." He coined the term in his 1976 book The Selfish Gene, where he used it to denote the transmission of cultural or societal habits from one person to another via non-genetic means. As such, he's the foremost authority on what a meme is and how it comes to be.
Donning a flower-patterned shirt and his usual irreverence, Dawkins gives us the nutshell explanation of memes as cultural replicators, but it's the illustrative video that follows his presentation that you'll really want to see. It commences just before the five-minute mark and is a true audio-visual feast.