Currently there are three different types of light sources:
nvsg::DirectedLight A directed light has no position, only direction. It represents a distant light source with parallel rays, such as the sun for example.
nvsg::PointLight A point light has a position in space, but no direction. The light rays emanate from it's position in every direction, such as from a light bulb for example.
nvsg::SpotLight A spot light has both position and direction. The light rays emanate from the position in only a limiting cone around it's direction.