ScreenSaver:A screensaver is a type of computer program initially designed to prevent "Phosphor burn-in" on CRT and plasma computer monitors by blanking the screen or filling it with moving images or patterns when the computer is not in use.
On Mobile phone it plays a very important role for the user by saving the valuable battery charge.When the phone is in idle mode i.e. user is not interacting with the device screensaver is activated. It is one of the important part of look and feel of the phone. Wiki