VNAV SPD - Aircraft will descend at the FMC programmed descent speed with the thrust levers closed and will disregard the vertical profile.
VNAV PTH - Aircraft will descent at approx the FMC programmed speed, and will maintain a vertical profile. To achieve this the thrust levers might be advanced to increase thrust to reduce the VS if the aircraft is going to be low on the profile (due headwind usually). Likewise, if the aircraft is unable to achieve it's descent profile (due tailwind usually), then the VS will be increased and you can expect the airspeed to increase beyond that programmed in the FMC. In this case, on B737 Classic, the FMC will advise Drag Required (speed brake) when the airspeed exceeds 10kts above the FMC speed in order for the speed to be managed with the descent.