Subsurface Scattering is the effect of light not getting directly reflected by a surface but penetrating it and bouncing internally before getting absorbed or leaving the surface at a nearby point. In short, light is scattered.

The RGB channels of a surface color can have different scattering values, depending on the material; for example, for human skin the red component is more scattered (as a rough approximation, you could say that the values for the three channels are blue = 1, green = 2, and red = 4).

In Cycles, a true Subsurface Scattering node has been introduced in Blender 2.67. Since Version 2.72, it also works with the GPU (only in the Experimental feature set).

But sadly, it still has the common big Cycles problem—it takes a lot of samples to produce a noise-free rendering. In short, it's slow.

Besides the true node, there are other ways to simulate Subsurface Scattering in Cycles. All the recipes in this chapter faking the SSS effect use the Translucent shader node to achieve this effect, and shifting of colors is simulated by giving a main color to the translucent component. Keep in mind that even if the scattering effect in the true SSS node could be basically considered a sort of translucency effect, these tricks are not comparable to the real Subsurface Scattering effect. They are just ways to give the impression that light is being scattered through a material surface.

Also, depending on the recipe, you'll see that the effects of Subsurface Scattering can be quite different, and the more suitable method should be used according to the type of material you are going to create. The differences in these recipes are basically in the way translucency mixing is driven by different types of input.