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

Follow me at euQfiz Digital