Ubuntu on mobile can be in the current developer preview parallel to install Android on an Android Smartphone. On Android side various Android versions are supported by the open source variant AOSP. However, only Android 4.2 currently supported.
About an app that exists for both Ubuntu and Android, you can switch between operating systems. The Smartphone will boot after a restart in the most recently selected system. However Canonical advises inexperienced users explicitly from the installation from, because it may well it might be necessary to flash the phone again if something goes wrong.
The actual installation of Ubuntu on mobile with the dual-boot option is quite simple: given only the dual-boot app for Android has to be installed. This will take care of installing Ubuntu and starts the new Smartphone, so Ubuntu instead of Android will be charged. In this case, the whole thing officially runs only on Google's Nexus 4, because other devices are not currently being tested.
