Is it possible to have an application run on a device in such a way that it is the only application that can ever run and also prevent the user from using the operating system at all? Tapping on the Home key or Back button would not exit the application and allow the user to have access to anything. If the device boots up, only this application would run.
This would be desirable in situations where devices are installed at a business for point of sales purpose or possibly where the device acts like a terminal in public places.
You can achieve what you’re describing by writing your app to replace the home screen (Launcher). From there, you control what other apps will run.
The Android SDK has a working Launcher project you can start from.
Be careful to allow some method of running a more powerful app (even if it’s just enabling ADB access) — otherwise you could leave your device in a state of needing a factory reset before it can be modified.