A meme is necessarily a joke but a joke is not necessarily a meme. A meme often, but not always, has a visual component to it. A meme is a cultural phenomenon. It is shared virally and evolves in the process.
EDIT: After reading the Wikipedia page like the two people that replied to me, it is now clear to me that a meme is not necessarily a joke.
EDIT: After reading the Wikipedia page like the two people that replied to me, it is now clear to me that a meme is not necessarily a joke.