When I first started doing visual effects I didn't fully understand shading. At first glance it seems like a relatively simple part of the animation pipeline. Assign a colour or texture to your object, adjust the materials specularity and your done. This isn't the case and at Carse & Waterman I spend the vast majority of my time creating and tweaking shaders. Shaders are the most important aspect of the rendering pipeline and is responsible for producing the desired result, be it cartoon or photoreal.
Lets have a look at a rather complicated aspect of shading - Subsurface Scattering (or SSS). SSS is the physical phenomenon where light enters a surface and scatters exiting at a different point. Most objects don't have a SSS component. Light hits the object and majority of the light is bounced back - usually around 70-80%. Some materials do exhibit SSS, most commonly plastic, marble, wax and skin.
Lets take a look at a relatively simple SSS example - orange juice. While SSS is typically found on hard objects, it can also be found in liquids like orange juice and milk. I've made a simple scene consisting of 2 glasses filled with orange juice. Both geometry is the same and the shaders are identical. The only exception is SSS. The orange juice on the right has no SSS, while the one on the left has SSS. Notice how the right glass looks like it is filled with orange paint.
SSS is only a minor part of the shading pipeline but I hope this demonstrates how important shaders are for the final look of an animation.