PAR is Photosynthetic Active Radiation which are the frequencies of light used for photosynthesis. This is light between 400 and 700nm and is the only parts of the light spectrum used for photosynthesis. These PAR values correspond to the wavelengths of the lights in an LED fixture.
PPFD is Photosynthetic Photon Flux Density and refers to the amount of PAR that actually arrives at the plant and is measured in micromoles per square meter per second (μmol/m2/s).
You really can't control the PAR. The lighting manufacturer does that when they select the wavelenghts of the chips. The distance from the plant determines the PPFD (how much of the PAR they receive).
Personally, although I understand it (a little!), I've never bothered to measure the PPFD. I start with the manufacturer's recommended distance from the plant (assuming they did the research on PPFD!), then either move the light closer or back it off depending on how the plants respond with the goal to get them as close as possible to the light without them showing any negative reaction.
I hope that helps!
K
Since I wrote the above response on a LED thread, I thought I'd add one more "technical" concept and then break it down in some "layman's" terms...
The one additional technical concept is called the Inverse Square Law and represents the "nemesis" for us indoor growers. The Inverse Square Law basically says that if you increase the distance between the light source and the plant by "X", the amount of light reaching the plant decreases by 1/x-squared (raised to 2nd power). So, the amount of light reaching your plants at 2' away is 1/4th of the amount of light reaching the plants at 1' away. Twice the distance is 1/4th the amount of light, 4x the distance would be 1/16th the amount of light, etc.)
So, putting some of these concepts together...
Where HPS, MH, CFL, etc can all be used to grow MMJ, many of the watts you're paying for are for parts of the spectrum (frequencies/wavelengths) that are not even used by the plants since they fall outside of the PAR range of 400nm-700nm. One of the benefits of LED is that you're only getting PAR values that ARE usable by the plant. So, more of the money you spend on your electric bill actually is usable by the plant.
So, what's the PPFD? Well, even at the same distance, the PPFD changes based on the wavelength of the light. Some wavelengths can travel further distances before they start to "fall off", however, thanks to the Inverse Square Law, we know that the closer we can get the plants to the light, the more PPFD we can get to the plant.
Where the HPS & MH will beat an LED is in penetrating the canopy, so the light can be further away to get the usable wavelengths lower in the canopy, but you're still sending parts of the spectrum that are not usable and have no bearing on photosynthesis.
So, the only thing to watch is that you don't have the light source so close to the plants that they show signs of light or heat stress. Some plants may tolerate more than others, but the most they can tolerate without compromising the plant health (or, possibly the lighting footprint to other plants!), the better!
Luckily, when we grow indoors, we're typically not growing 10' plants. In a 7' tent with the light 1' from the ceiling, 1' from the canopy, and a 1' tall container, you're gonna get a plant no more than 4' tall, or so. Today's LED technology can easily penetrate those smaller canopies, get more usable spectrum to the plants, and allow the grower to place their plants close to the source without negative effect.
I hope that helps some others!
K
But since too many customers are asking about it, she decided to give it a try. Mars Hydro design is a little different. Now we might have something to look forward to. 
And speak of what, anyone know which strain can cure insomnia? 
