I've actually seen a Theremin in person, and watched a demonstration of how it works.
The Theremin is an electronic instrument played without touching. An electromagnetic field is generated between two antennas, and sound is created as the hand moves between the two.
I've never thought of the Theremin as a concert instrument, per se. I've heard it used to create--dare I say, "weird"--effects in movie scores and the like. But there's at least one person who considers the Theremin a serious musical instrument.
Her name is Carolina Eyck, and she is a "Theremin virtuoso". Who even knew there is such a thing? At any rate, she has a new disk out, "Improvisations for Theremin and Piano". I've only heard one selection from the disk, and you can hear it too when you watch the video.