SMT is the process
Surface-mount technology describes printing paste, placing components on pads and reflowing the assembly. It applies to any package that can be soldered on the surface.
SMD is the part
A surface-mount device is a component designed for that process: chip resistors, SOICs, QFNs, BGAs, and so on. The package determines the footprint, the stencil aperture and the inspection method.
Why the distinction matters
When a drawing says "SMT assembly", it describes the process; when the BOM lists an SMD, it describes the part. A mixed board needs both processes documented, and the through-hole side must not be reflowed with parts that cannot take the profile. That documentation is also what a pcb assembly services provider reads first, because the stencil, the profile and the inspection method all follow from it.
