Does anyone know what this is?

I ran across this on you tube. Does anyone know what it is?

I've seen one of those before. Serj Tankian from System of a Down was playing one. I have no idea how they work though.

Here is another video on how to make one.

It's a theremin. They're really interesting, but I can see them being hard to master.

EDIT: the second video already cleared up what it's called, but yeah.

Theremins are used by a variety of guitarists I read an article on guitar world about it recently. They were used in 50s horror and sci-fi movies.