PAR! What is PAR? PAR stands for Photosynthetically Active Radiation. Now, what is that exactly? PAR is the exact spectrum of wavelengths found in light that enables Photosynthesis. Without light, plants would have a harder time producing photosynthesis which is necessary in order for them to grow. Photosynthesis is a process used by plants, algae, and certain bacteria to use the energy provided by the sun to turn it into chemical energy. In order to discuss PAR, we're going to break down the two types of Photosynthesis as well as the spectrum of waves this process falls under. 
The spectrum of wavelengths is 400 -700 nanometers, now Infrared and Ultraviolet do fall above and below this spectrum however, they both hold beneficial properties for plants and many grow lights will have Infrared or Ultraviolet within their diodes. The two types of Photosynthesis are Oxygenic and Anoxygenic Photosynthesis.