what is the different between the throttle body adapter and the spacer?

 

You mean between the spacers and the gasket?

 

Spacers allow more air to be in the IM.

Gaskets reduce heat coming from the block. Keeps the IM cooler.

They have both TB gaskets and IM gaskets.

 

I believe the difference between the two is; the adapter plate adapts your IM to a different size TB, which does ultimately allow more air. The throttle spacer is usally the same size bore (air opening) but acts as an insulator to keep the air from heating up as much before reaching the IM. Some have a smooth bore and others have a helix spiral bore. Cooler air is denser air which helps your fuel to burn much better. I think both achieve the same cooling effect but with different features (Adapter= larger air volume & the Spacer= inlet for NOS or Vacuum.)
I hope this helps a little and most of all, I hope I got it right!