Almost any surface will scatter light from a surface in various directions. Surfaces that possess a slight translucency will scatter some of that light below the surface, sometimes changing its color by absorption, and spreading the light further. You can see this in your own skin: if you hold up your hand in front of a strong light you will see the red almost glow on the thinnest part between your fingers. This Sub Surface Scattering is common in most real life materials to some extent, but only needs to be considered in a 3D scene for the obvious candidates such as skin (not just human), wax, or certain liquids like milk. Sub Surface Scattering in Blender will require some extensive processing and therefore will increase render times. This is particularly true when combined with other render-intensive processes such as ray transparency, reflection, and Ambient Occlusion.
Ambient Occlusion, or AO, simulates the soft shadows found on objects when illuminated...