'Memes' are internet crazes - pictures, animations, videos - that are usually sent via email.
Perhaps the most famous are the LOLcats, pictures of cats with deliberately ungrammatical and misspelled captions, telling you what the animal would be thinking if it were human: 'I can has cheezburger?'
Meanwhile, people around the planet are dancing something called the Harlem Shake. Not all the nonsense circulating the internet is a true 'meme'. Technically, the point of a 'meme' is that it is not just passed on but imitated.