What Are Slip-Fit & NPT Bath Spouts?

Slip-Fit Spout: 

This spout has an internal assembly that slips over a copper pipe. This spout has an opening on the bottom side where a setscrew is tightened to hold the spout onto the copper pipe.

NPT Spout:

NPT refers to a standard thread size in the plumbing industry. This spout threads onto a copper pipe. This spout also has an opening on the bottom side, but there is no set screw, the copper pipe would be visible inside the opening.