An electronic light source converts electrons into photons. Lights for human vision measure output with lumens (measured in lux or footcandles). Lights for photobiological reactions measure output with photosynthetic photon flux (PPF) or the rate at which the light source emits in the photosynthetic region.
Another way of stating it is: PPF tells us how much PAR a light source emits.
Where PPF measures total PAR output of a lighting system, Photosynthetic Photon Flux Density (PPFD) measures how much of that light is delivered to a given area.
For further information refer to our article and link to a video on PAR.