Dictionary.com defines meme as "a cultural item that is transmitted by repetition in a manner analogous to the biological transmission of genes". Basically that means a meme is something that gets spread around society via things like social networks and grows, develops, and mutates as it spreads.
Some classic examples of memes are "planking" (lying flat in a random place like a plank):
Then there was "Batmanning" (hanging upside down like Batman):
Then it was "owling" (perching like an owl):
Then it started getting more complicated with "Hadukening" (posing like u just got hit by a fireball in Street Fighter):
Which lead to "Vadering" (posing like you're being Force Choked by Darth Vader):
And the newest way to prove you have nothing better to do with your time is.... "QUIDDITCHING" (posing like you're playing Quidditch in the Harry Potter universe):
Although like most memes, it will be EVERYWHERE then disappear to make way for the next meme which is going to be...