From Wikipedia:

The NES Zapper, also known as the Beam Gun in Japan, is an electronic light gun accessory for the Nintendo Entertainment System (NES) and the Japanese Famicom. It was released in Japan for the Famicom on February 18, 1984 (1984-02-18) and alongside the launch of the NES in North America in October 1985. The Famicom version of the Zapper, made for the game Wild Gunman, resembled a revolver-style handgun, but the North American version resembled a futuristic science fiction ray gun that also tied into the design of the NES. Early versions of the Zapper were a dark gray, but its color was later changed to orange, as was now required for all "toy guns" under U.S. Federal Regulations. Although originally included in some configurations of the NES, the Zapper was available for purchase separately.

The Zapper allows players to aim at the television monitor and shoot various objects such as ducks, clay pigeons, targets, cowboys, or criminals, or other objects. Pulling the trigger emits a loud, characteristic noise akin to a combination of a high-tensile spring snapping and a mechanical "click".

History and usage
The Zapper was first released in 1985 with the launch of the NES in North America. It came bundled with the NES console, the Robotic Operating Buddy and two games Duck Hunt and Gyromite.

The Zapper is used on supported NES games such as Duck Hunt and Wild Gunman. To hit targets on the screen, players point the device by lining up the front and rear sights with the desired target and pulling the trigger. The Zapper could also be used on the title screens of games to move the cursor done by pointing the device away from the screen and pulling the trigger or starting the game (pointing at the screen and pulling the trigger).

When the trigger on the Zapper is pressed, the game causes the entire screen to become black for one frame. Then, on the next frame, the target area is drawn in all white as the rest of the screen remains black. The Zapper detects this change from low light to bright light, as well as which frame the change was detected. This is how the game knows which target has been hit.After all target areas have been illuminated, the game returns to drawing graphics as usual. Another technique involves making the entire screen black in one frame and white in the next. Calculations are used to determine electron beam position. This technique works only on conventional CRT television sets; modern plasma or LCD screens are incompatible with this method.In either case, the whole process is almost imperceptible to the human eye, although one can notice a slight "flashing" of the image.