LoopStack is an app that lets you make music by recording loops into your Android device. This is the free version, which comes with some serious limitations, but does work as a reasonable demo of the full app.
Four tracks or more
The simple interface has four spaces to record your loops. Simply hit the REC button on any of them to record whatever sound you want. Each loop comes with a separate volume and reverb control, so you can do some basic 'mixing' of your track as you build it.
LoopStack lets you set the BPM, beat measures and time signature. You can also mute the metronome if you want. As the developer says, it's essential to use headphones with the app to prevent it from recording itself, and keeping your recordings relatively clean.
This version allows you to record for five minutes, after which the app will reset. The save, load and export features are disabled unless you buy the full version. While LoopStack is perfectly functional, it doesn't do anything particularly special, and there are other totally free similar apps available like Looper.
Could be more intuitive
My main problem with LoopStack is the recording itself. Your first loop doesn't start recording when you start playing. Many loop apps will begin to record as soon as you make some noise, or at least give you that option, which makes building loops very fast. The time limit on this trial version also means you can't really get anywhere with recording unless you buy the full thing. Making a track in five minutes will be beyond most people.
While the LoopStack interface is simple, and there's a quick tutorial, it would be better to have more in-app instructions or guides for beginners. It also has not been updated since 2013, so it looks like the developer has stopped working on it.
Functional, but outdated
This trial version of LoopStack is very limited, and it really needs updating again to be truly recommendable.