YTPMV & 音MAD
YTPMV stands for YouTube Poop Music Video. This is when clips shows are spliced together to sync with music (usually from a video game) to make a video. These videos use a variety of sources, usually from cartoons or anime, but really anything can be used.
The audio portion of a YTPMV is sourced from various forms of entertainment, and each image or character on screen is in individual note that's a pitch-shifted sample of the audio from the film or video games scene being used to represent that note. A sense of pride is displayed when the editor makes YTPMV video set to the tune of a song, without using that song as a base (BGM - Background Music) for the YTPMv, yet still making it sound very similar to the original song
音MAD (Otomad) is basically the Japanese equivalent, although not always quite the same, as Otomads can sometimes be clips spliced to make a small joke. For more information on MAD/音MAD (Otomad), read this archived link.
There is mostly no difference between the two terms (although 音MAD covers a wider base), but the easiest to define difference is that 音MAD are from Japan/Korea, YTPMVs are from the West.