A PMV, short for "Pony Music Video" a word play of AMV "animu music video", is a video using sounds from My Little Pony or, more commonly, external music to external music clips using clips from the external music. Most PMVs try to have the ponies lip-sync the lyrics to create an effect as if they themselves are actually singing, but others do without it.
Some PMVs have gained considerable attention, with one being the focus of a collage paper discussing Fair Use, and another for acutally beint Tweeted by Wierd Al himself.
The Three Stages of PMV editing
Some PMV's can take countless hours of constant work, and the spectrum of effort put into these projects can be pretty wide.
A PMV editor cuts out sequences from the show, often to lip sync them with the characters from the show. These PMV's can take maybe about a night's work of effort to compile. Pretty easy. Such examples of this include the most popular of them all - You're Gonna Go Far Kid, which has over 11 million views currently.
Once you get bored of using the same fitting sequences from the show over and over, it would be about time that someone improves their editing skills, often with the use of After Effects, and starts building templates cut-out from the characters from the show. This greatly improves the flexibility of what one can do with a PMV. It also often has selected music that is often less mainstream.
Some PMV's can be inspired by animated concept art, such as the following PMV below, "The Garden", which was inspired by Disney concept artist Mary Blair.
Those who put intensive amounts of animation and effort will only get this far. Often, Stage 3 PMVs are animated completely from scratch, sometimes have original music, and have very few clips or templates (if not, then no templates) actually taken from the show. These animations tend to be immune to Youtube's Content ID filter, so artists usually do not have to worry about their PMV's getting DMCA'd at this point.
Some PMV animators can take their art to come up to par with the show itself, such as JanAnimatons' Picture Perfect Pony .