Area Light
If you could turn a four-point polygon into a light, you'd have an Area light. Light is sent out equally in all directions, except along the edges. This type of light might be used for flat light panels or as a portal light to bring GI samples inside a room.
Unlike the Distant, Point, and Spotlight lists, you can size these lights, just as you would an object. Another distinction is that ray-traced shadows will have physically accurate edges that are sharpest near the object that is casting them and get softer further away. A smaller area light will result in sharper shadows than a large area light.
Normalize
There is a Normalize toggle in most Light Properties windows. Light intensity is now set in Lux, which is Lumen per meter squared. This means that as your lights change size so does their intensity. With Normalize off, a smaller light of a given intensity will give out less light, a bigger light more. With Normalize on, the intensity stays the same. It might appear to become dimmer in some cases but this is merely because the light output is being spread over a larger area.
The area of a Distant light will decrease as you reduce the angle in the panel, and thus its power diminishes if Normalize is not checked. 0° is a special value, and the Distant light will behave the same Normalized or not, with hard shadows.
Portals
Light Portals.7z 
Area and NGon lights can be used as portals. Portal lights are guides placed in indoor scenes that help the renderer to concentrate environment lighting through openings like small windows.
Portals work with the environment light and its plugins - Image World, Textured Environment, gradient backdrop or similar. While portals will add time to renders they also clear up noise and bring a lot of extra detail that you don't get using an environment light alone.
