Unlike most music-creation apps, it doesn't take an existing musical instrument and cram it awkwardly into the iPhone's interface. Instead, it creates a completely new "instrument" designed specifically for the iPhone. Essentially, you're provided with a colored screen and a quiet drone. As you tap the screen in various places, different tones play depending on where you tapped. They then loop, creating a unique piece of music on the fly, one that changes gradually on its own once you stop tapping.
You can also just let it create music on its own, coming up with a new, unique piece every time you run it. It's also beautiful, with the tones appearing as colored spots that slowly fade. It's as satisfying to actively play with as it is to let do its own thing.
Bloom is available on the App Store now for $3.99, and if you're a fan of ambient music or music in general, you'll be wanting to get it right now.