Unity supports different Rendering Paths. You should choose which one you use depending on your game content and target platform / hardware. Different rendering paths have different performance characteristics that mostly affect Lights and Shadows. See render pipeline for technical details.
If the graphics card can’t handle a selected rendering path, Unity will automatically use a lower fidelity one. For example, on a GPU that can’t handle Deferred Shading, Forward Rendering will be used.
Deferred Shading is the rendering path with the most lighting and shadow fidelity, and is best suited if you have many realtime lights. It requires a certain level of hardware support.
For more details see the Deferred Shading page.
Forward is the traditional rendering path. It supports all the typical Unity graphics features (normal maps, per-pixel lights, shadows etc.). However under default settings, only a small number of the brightest lights are rendered in per-pixel lighting mode. The rest of the lights are calculated at object vertices or per-object.
For more details see the Forward Rendering page.
Legacy Deferred (light prepass) is similar to Deferred Shading, just using a different technique with different trade-offs. It does not support the Unity 5 physically based standard shader.
For more details see the Deferred Lighting page.
Legacy Vertex Lit is the rendering path with the lowest lighting fidelity and no support for realtime shadows. It is a subset of Forward rendering path.
For more details see the Vertex Lit page.
|Deferred||Forward||Legacy Deferred||Vertex Lit|
|Per-pixel lighting (normal maps, light cookies)||Yes||Yes||Yes||-|
|Realtime shadows||Yes||With caveats||Yes||-|
|Depth&Normals Buffers||Yes||Additional render passes||Yes||-|
|Light Culling Masks||Limited||Yes||Limited||Yes|
|Lighting Fidelity||All per-pixel||Some per-pixel||All per-pixel||All per-vertex|
|Cost of a per-pixel Light||Number of pixels it illuminates||Number of pixels * Number of objects it illuminates||Number of pixels it illuminates||-|
|Number of times objects are normally rendered||1||Number of per-pixel lights||2||1|
|Overhead for simple scenes||High||None||Medium||None|
|PC (Windows/Mac)||Shader Model 3.0+ & MRT||All||Shader Model 3.0+||All|
|Mobile (iOS/Android)||OpenGL ES 3.0 & MRT||All||OpenGL ES 2.0||All|
|Consoles||XB1, PS4||All||XB1, PS4, 360, PS3||-|