From the instuction manual:
The Zapper is a new type of light gun that challenges your reflexes and quick judgment. It combines a light-sensitive gun with video-game fun.
The Zapper, or Light Gun as it is called by many people, has been around almost as long as the NES itself. After the Robotic Operating Buddy (R.O.B.) failed as an accessory, the Zapper was bundled in the Nintendo Entertainment System Action Pack along with the control deck, two NES controllers, and the Super Mario Brothers/Duck Hunt game cartridge. By detecting light on the TV screen, the gun could tell what it was pointed at and register the shot as a hit or a miss. Although it's difficult to imagine the NES without the Zapper, there were relatively few games that it could be used for, although compared to the R.O.B., it had a huge library.
Complete list of NES games that use the Zapper, either optionally or as the main controller: