Quick tutorial for you as it gets asked quite a bit. This is by no means a definitive guide simply enough information to get you up and running.

Render out multiple passes in maya as an OpenExr file then composite them in nuke. Tutorial aims to get you up and running with multipass rendering in Maya as quickly as possible. In this example we render out and comp an ambient occlusion pass a diffuse pass a specular pass and a shadow pass.

Multipass Rendering in Maya Composite in Nuke - YouTube

You might want to change this to HD in youtube

n'joy
Dave