Taken from Wikipedia:

The term "Internet meme" refers to a catchphrase or concept that spreads rapidly from person to person via the internet, largely through email, blogs, forums, social networking sites, and instant messaging. The term derives from the original concept of memes, although it has come to refer to a much more narrowly defined category of cultural information. Examples include Rickrolling, lolcats, "Leave Britney Alone", Keyboard Cat and "All your base are belong to us".