"A lip dub is a type of video that combines lip synching and audio dubbing to make a music video. Usually made by filming a group of people lip synching while listening to a song or any recorded audio then dubbing over it in post editing with the original audio of the song. Lip dubs can be done in a single unedited shot that often travels through different rooms and situations within a building, campus or even a city."
I first saw a lip dub in 2009 or perhaps early 2010, it was sort of a takeoff on flash mobs. Recently I was sent the link to a truly amazing lip dub done in Grand Rapids, Michigan and set to Don McLeans American Pie.
If after watching you want to know more, here is the Making of GR LibDub.
Of course there are now hundreds of variations on the web, my personal favorite is LipDub Sorbonne Paris - Coldplay Viva La Vida.