There are different ways to control the lights to make them do smart things. You probably have one installed on your smartphone already that you control your lights with. Apps you develop could do anything from simply changing colors or switching on and off in response to a user input, or they might be linked to some sort of sensor or other input, such as switching on when a sound is picked up or a tweet is received. They are not limited to smartphone apps. It’s basically everything that makes use of the Hue system APIs.