A meme is a thing that takes on a life of its own and evolves within the human population, like a gene that people carry and pass on to other people. The O RLY owls are a perfect example of that, someone started it with a picture of an owl with a caption and it took off from that - a lot of people started making them, then they started captioning other things like cats, politicians, etc. Now stuff like the Numa Numa guy, the Star Wars kid and Tay Zonday are considered "memes" although that bastardizes the original meaning a bit.
The other day a guy asked me how windchimes worked. I processed that for about 2 seconds, and then I said to myself, "Holy shit, I have to make some changes in my life."
-clerk at Salvation Army thrift store