The term always seemed self explanatory, but I just realized I have no idea what Shading Domains are all about.  I guess I have three basic questions...

What defines a shading domain?  Is it unique to an object or some way of keeping calculations for one shader from affecting another?

Is it something I need to understand?  Are shading domains a form of scene optimization, or something that allows you to create specific materials?

Is this something unique to Carrara, or perhaps called by another name in other programs?

Its the way to divide a surface of an object as many shaders you need. In Lightwave is called surface. Others softwares use other nomemclatures.

Imagine a beach ball with many colors: you can have a shade domain for every color. In truth its a group of polygins that will receive a shader, so you can have a shade doamin with glass shader, other with a wood shader and so on.

This is somthing that you need to know to model and shade things in Carrara or any other software:

http://youtu.be/3g-f5or3i00

http://youtu.be/hYK5cpCyzP8

http://youtu.be/AlGi18u15go

So, like a mat group in Poser?

Yes! The same thing.