The word meme was coined by Richard Dawkins in his 1976 book The Selfish Gene to refer to cultural behaviors and beliefs that were transmitted from individual to individual, and eventually, from one culture to another. Etymologically, meme (like memory and mimeses and a number of other words in Modern English) is cognate with Greek mimema or "something imitated." Dawkins used his new word to discuss the way ideas and cultural innovations spread from person to person, and culture to culture. Dawkins used examples ranging from melodies, or religious beliefs and concepts, to clothing styles, and technological advances. Dawkins' basic concept, and the idea of memes spreading virally, was fairly quickly applied to the Internet, after the Web became popular.

Internet memes initially arose almost spontaneously, as an image, phrase or motif spread from user to user. Often they were derived from original content posted to the Web, especially amateur videos. Often images are slightly edited. Image Macros, with superimposed text are common meme-fodder. Pop culture, particularly "cult following" TV shows and films, like Star Trek, Star Wars, 300 have briefly been the source of memes. Although it pains me to admit it, the best discussion I've seen of Internet memes is on WikiPedia.

4Chan's Caturday now better known as LOLCats (now associated with ICanHasCheezeburger) is an example of an image macro meme. There's an LOLCats image macro at the top of this post. Here are some other memes.

Chuck Norris Facts (and a pretty good discussion on WikiPedia)

AYBABTU (All Your Base Are Belong To Us)

Rick Rolling (Another 4Chan innovation; it involves someone tricking you into clicking a link expecting one thing but in fact, the link goes to a video of Rick Astley singing "Never Going to Give You Up").