N90 and N95 are the protection levels in the national mask standard. The different standards represents different protection-  and filtering capabilities of a mask, i e its ability to block dust, bacteria and viruses.

The filtering capacity of N90 is lower than that of N95. The N90 mask can block 90% of the dust.

N95 offer better filtering capacity and may filter away more than 95% of dust, bacteria and viruses.

So the difference between N90 and N95 lies in the ability to block dust and particles.