For those need to keep hitting the books like me, here are the basic definitions.
http://www.arlon-thermal.com/thermal_prop/
Just quickly:
Thermal conductivity, the rate of passage of heat / time, thru a material
Thermal resistance, the insulation value of a material
Thermal impedance, material resistance values plus contact resistance values
You add it all up what do you get? Impedance. What do you want? Lower impedance. When do we want it? Always. We don't want a breakdown of any of the materials, but beyond that, we don't want to impede any heat flow, and we want the fastest outflow.