Which is kind of oxymoronical since the theremin is an instrument that creates errie, spooky sounds without actually touching the instrument, and you have to touch the iPhone-theremin version. Think sci-fi movies from the 50s. Also, there's a great band Octopus Project that features a theremin. [1.37 seconds in to see/hear a theremin in action].†
Theremin players position their hands between two sets of antennae -- one hand oscillates the frequency, the other hand controls the volume.
Thereminator has a large gray "control area" where the droning sounds are created. Below that is the pitch slider. On the left is the volume slider.
You select one of the four wave forms and start pressing and sliding your fingers in the gray area to create different weird-o sounds ... That's the fun part, mixing and matching wave forms with the echos and delay effects with one or several fingers.
It is a very fun app. available in iTunes for $2.99.