Night vision cameras, also called IR cameras, employ a very sensitive digital chip that can capture scenes in very low-level lighting conditions. During the day, the camera takes images in color while at night it automatically switches to black-and-white mode when the light level drops a certain amount. These cameras do need some light in order to take images, even if it is the light of the moon or stars.
Some people confuse day-night cameras with infrared cameras but they are not the same. The latter use infrared illumination for their operation to see without any light at all.