WTF?
YTPMVs, or YouTube Poop Music Videos, are an evolution from the original YouTube Poop (YTP). Put simply, a YTPMV is when you pitch shift any given sounds ("sources", usually anime girl screaming scenes) to match the tune of an existing or new song. In a YTPMV, great emphasis is placed on making the result sound nice, and often the result is more pleasing to listen to than the original song.
YTPMVs have had an important role in popularizing chiptune (keygen) music, which are widely thought to lack in sound character and sound "piercing" to the ear.