Probably one of the most random youtube trends ever. Basically a bunch of people sit and pretend not to notice one person dancing absurdly in a costume so the song "Harlem Shake." Then, at a certain point in the song, everyone just starts dancing obnoxiously in absurd costumes. There's no rhyme or reason to it - it's just funny.
So originally, a few kids from Australia put up a version of this and it became instant Internet gold, with everyone recreating it in a certain scenario. The most popular ones are in schools, but the funniest are the most obscure, like the news anchor and army baracks ones. Want 5 minutes of internet fame? recreate the most absurd version of this video. Goodluck betches.
Here's a graphic showing just how popular the Harlem Shake got and how much time crazy people like us spent watching them.