Choosing a Rendering Path
Unity supports a number of rendering techniques, or ‘paths’. An important early decision which needs to be made when starting a project is which path to use. Unity’s default is 'Forward Rendering”.
In Forward Rendering, each object is rendered in a ‘pass’ for each light that affects it. Therefore each object might be rendered multiple times depending upon how many lights are within range.
The advantages of this approach is that it can be very fast - meaning hardware requirements are lower than alternatives. Additionally, Forward Rendering offers us a wide range of custom ‘shading models’ and can handle transparency quickly. It also allows for the use of hardware techniques like ‘multi-sample anti-aliasing’ (MSAA) which are not available in other alternatives, such as Deferred Rendering which can have a great impact on image quality.
However, a significant disadvantage of the forward path is that we have to pay a render cost on a per-light basis. That is to say, the more lights affecting each object, the slower rendering performance will become. For some game types, with lots of lights, this may therefore be prohibitive. However if it is possible to manage light counts in your game, Forward Rendering can actually be a very fast solution.
In 'Deferred' rendering, on the other hand, we defer the shading and blending of light information until after a first pass over the screen where positions, normals, and materials for each surface are rendered to a ‘geometry buffer’ (G-buffer) as a series of screen-space textures. We then composite these results together with the lighting pass. This approach has the principle advantage that the render cost of lighting is proportional to the number of pixels that the light illuminates, instead of the number of lights themselves. As a result you are no longer bound by the number of lights you wish to render on screen, and for some games this is a critical advantage.
Deferred Rendering gives highly predictable performance characteristics, but generally requires more powerful hardware. It is also not supported by certain mobile hardware.
For more information on the Deferred, Forward and the other available rendering paths, please see the documentation here.