PAR, or Photosynthetically Active Radiation designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye.
PUR, or Photosynthetically Usable Radiation is that fraction of PAR that is absorbed by zooxanthellae photopigments thereby stimulating photosynthesis.
Photosynthetic zooxanthellae in corals and some other marine invertebrates utilize light in the 400 to 700nm range in different ways. The following from the Aquarium Lighting article on the American Aquarium; Aquatic Information, Products site relates to the graphic above:
"Important Definitions as it applies PAR in plants and zooanthellic algae: See the graph above as it corresponds to each of these definitions.
*A: Phototropic response; having a tendency to move in response to light. Basically this is the Chlorophyll containing plant or algae "moving" to respond to a positive light source to begin the process of photosynthesis (initial growth of plants, zooxanthellae, etc.).
*B: Photosynthetic response; the process which begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls.
*C: Chlorophyll synthesis is the chemical reactions and pathways by the plant hormone cytokinin soon after exposure to the correct Nanometers wave length (about 670 NM) of light resulting in the formation of chlorophyll, resulting in continued growth of a plant, algae, zooxanthellae and the ability to "feed" and propagate, and without this aspect PAR (670 NM light energy), zooxanthellae and plants cannot properly "feed" propagate. The results of the lack of this high PAR "spike" would be stunted freshwater plant growth, and eventually poor coral health in reef tanks."
As you can see, a light source which provides light in all three of these zones (A, B and C) is important for coral growth and health.
Read on to see the light spectrums of some of the most popular fluorescent reef tank lights.