Emission, or glow, can make an object appear to emit light. This is useful when you need a textured effect like lit panels, circuitry, lava, or other complex effects with local influence in lighting. Since these materials do not truly emit light, indirect lighting is used to achieve the effect that this material is influencing its surroundings. No bump or normal mapping is available because the effect would not be seen either here or in reality.
This is the result of a flat color or a signal from a texture or procedural pattern used to drive the effect. The color black is essentially "off" or no effect.