Passive (often called 'starlight vision') nightvision equipment simply amplifies the available light and will not work in absolute darkness. It is similar in principle to adjusting the contrast (difference between light and dark) of a picture.

Active nightvision, like the type found on many new camcorders, actually projects an near-infrared light source out. The optics in the camera are sensitive to this spectrum, but it is just out of visual range so you can't see it.