Let there be light
Delta light sources:
Point light: An ininitely small area light source chraracterised by its position and radiant intensity.
Spot light: An infinitely small light source characterized by two angles the inner cone and outer cone, its default orientation is pointing down the y axis.
Infinite light sources:
Distant Disc light: An infinitely distant spherical cap characterized by its radiance and the angle subtended from the view point of an object. The sun subtends an angle of about 0.52 to 0.54 degrees as seen from earth http://er.jsc.nasa.gov/seh/math30.html.
Dome light: An infinitely distant sphere that envelops the scene with a HDRI texture characterized by spherical angles phi(horizontal) and theta(vertical) corresponding to the amount to rotate.