3rd Year: AOV's & Render Layers

Old Man 4K Head Render, Model provided by: (Eat3D) 

So Aov's (Arbitrary Output Variables) work like render layers in other renderers, allowing you to render out certain layers in order to tweak a render to an individuals liking.

Beauty Pass

The Beauty Pass is all of your other layers at work put together, to the ambient occlusion, specular another layers will all come together in order to make your final render. In render settings you can choose what layers you can render and which one to leave out, so ambient occlusion, diffuse direct, diffuse indirect, specular direct, specular indirect, SSS (Sub-Surface-Scattering) direct and SSS indirect will be the other layers used to create this image. These are not all the Aov's, but these are the main ones.

Diffuse Direct Pass

Diffuse Indirect Pass

Specular Direct Pass

Specular Indirect Pass

SSS (Sub-Surface-Scattering) Direct Pass

SSS (Sub-Surface-Scattering) Indirect Pass

So once your Aov's are set up, hit the renderer on either 'Render View' or 'Arnold RenderView', save the file out as a multi-layer EXR.

Adobe After Effects CC 2017 (EXtractoR)

Once brought into after Effects create your composition, the proceed to use
Effects > 3D Channel > EXtractoR, in order to extract the different layers. The one huge benefit of this is that it means you can tweak all of the layers like specular or sss individually, so you can get a really good idea of how to make the render as good as possible.