Night vison goggles generally are one of two types. One type of night vision goggles pick up any light in the given area and amplify it several hundred times. This type is commonly used for war reporting, tinting the picture green. The other common type of night vision goggles are heat-sensing/infrared goggles. These show anything that emit heat signatures in the given area. There are currently goggles that combine both of these technologies to create images that are almost as clear as day.
Night vision photograph from Operation Iraqi Freedom