Photosynthesis is using Light Energy to convert CO2 and water into oxygen and carbohydrates such as sugar or starch.
Cellular respiration is using oxygen to convert biochemical energy into ATP (Energy) and CO2
The relationship between the 2 is that what the 2 creates and let out is what the other needs to work like photosynthesis creates oxygen as a byproduct and to do cellular respiration, it needs oxygen. Cellular respiration creates CO2 as a byproduct and to do photosynthesis, it needs CO2.