In forward rendering, each object is rendered once for every light in the scene.
"Deferred rendering takes another approach: first of all of the objects render their "lighting related infomation" to a texture, called the G-Buffer. This includes their colours, normals, depths and any other info that might be relevant to calculating their final colour. Afterwards, the lights in the scene are rendered as geometry (sphere for point light, cone for spotlight and full screen quad for directional light), and they use the G-buffer to calculate the colour contribution of that light to that pixel."
