Daylight or natural light is visible radiation which is generated by the Sun. It can reach is in three different ways:
Of the three, direct sunlight is the most powerful source. It provides not only visible light, but also ultra violet and infra-red (heat) radiation. A summer sun can provide as much as 1000 W/m² of radiation measured in a plane normal to the sun's direction.
When sunlight travels through the Earth's atmosphere, the light is scattered by water vapour, molecules and particles diffusing a certain proportion of it. Under normal conditions, this scattering is higher for short wavelengths (blue) than it is for long ones (red), which is why the sky dome appears blue.
When light hits any surface, part of it is reflected back. This reflection is usually diffuse (non-directional) and is dependent on the object's reflection. The reflection of the outside ground is usually in the order of 0.2 or 20%. This means, that in addition to the sunlight and skylight, there is also an indirect component which can make quite a significant contribution to the light inside a building, especially since the light reflected off the ground will hit the ceiling which is usually very bright.