NDVI uses the difference in wavelengths reflected by chlorophyll to detect living plants and distinguish them from green objects.

In the above image, the lighter pixels indicate a higher NDVI, indicating the presence of plant life. The pre-fire image shows thick vegetation spotted with clear cuts. The post-fire shows much of that vegetation gone.

NBR uses the difference in wavelengths reflected by exposed soil to detect burned areas. Using the difference between pre and post fire NBRs allows us to identify areas burned by the fire specifically, excluding areas that were already burned when the fire occurred.