Taken from Wikipedia:
Movies on YouTube that take clips from TV shows or movies and speeds them up, slows them down, and places them in a random order. The first "Youtube Poop" was created by SuperYoshi sometime in 2004 using an episode from the Super Mario Bros. 3 cartoon titled "Recycled Koopa". Contrary to popular belief, this means that the "Over 9000" video popularized the genre, it didn't create it. A very famous YouTube Poop is Kajetokun's OVER NINE THOUSAAAAAAND!!!", where he takes a clip from Dragonball Z in which Vegeta says "It's over 9000!!!" and plays it over and over, editing the clips. Other YouTube Poop artists include Superyoshi, Retrojape, NOTYaminomalex (formerly Yaminometamothamos), DukeofFortuneMan, CheezNapkin, JingWu, tetsuo9999, 1upclock, AntiEthan (formerly EATAN), mrsimonalt, and Tabull89.
I added in the part about SuperYoshi starting the whole thing to supress the faggotry a bit.