Though ThumbJam includes a high-quality library of sampled real instruments, from flute and violin to accordion and electric guitar, its interface does not attempt to imitate any of these. Instead, it is made up of a user-controllable number of parallel horizontal bars representing pitch. Part of the beauty of ThumbJam is how easy it is to jump in and play. Select an instrument, select a scale (a large selection of scales such as major, minors, blues, etc. is built-in, or you can create your own), and start playing!
ThumbJam is worth the price just as an excellent virtual instrument. However, its other features turn it into a powerful mobile studio. First is its looper. You can record your first loop to a metronome click, or just play, and ThumbJam will figure out your tempo. By default, the original loop continues under subsequent loops, which lengthen your composition if they are longer than the original, rounded up to the nearest bar. All that is easily adjustable in the settings.
If that isn't enough, ThumbJam even includes a sampler so you can import your own samples or record them right into the app. I'm looking forward to trying this with a middle school class in the Fall!
Here are two videos to give you a better idea of how ThumbJam works. This one uses an old version of the app, on an iPod, but gives you a good idea of what the app can do: