Rather than generating light like a CRTs, LCDs act as light values. Therefore, they are dependent on some external light source. In the case of a transmissive display, usually some sort of back light is used. Reflective displays take advantage of the ambient light. Thus, transmissive displays are difficult to see when they are overwhelmed by external light sources, whereas reflective displays can't be seen in the dark. You should also note that at least half of the light is lost in most LCD configurations. Can you see why?