I didn't know these kind of videos/songs were classified as YTPMV. What's the MV part though?
Basically, YouTube Poop Music Video.
Initially, it was a fad that spawned from the NicoNico MAD's, using sources that were initially used for YTP's (Zelda CDI, Pingaz, ect). Over the last four years though, it grew to be anything that included samples from a non-music related source pitched and timed to a song.
Personally, I don't like the term YTPMV. It just sort of stuck .-.
Kind of like how you say "roll down the window" even though no modern cars have windows that you have to actually roll down.