GPP refers to the total amount of energy that is captured by plants through photosynthesis in a given area or ecosystem.
NPP is the amount of energy that remains after plants use some of the captured energy for their own respiration.
Respiration is the process by which organisms, including plants, release stored energy from organic compounds to carry out life functions.
Photosynthesis is the process by which green plants and some other organisms convert sunlight into chemical energy in the form of glucose.