Nitrite is the second stage of the nitrogen cycle in your aquarium. It usually occurs after ammonia is converted by bacteria to nitrite. More specifically, when ammonia-oxidizing bacteria (AOB) oxidizes ammonia.
Nitrate, commonly confused with nitrite, is the last step in the nitrogen cycle of your fish tank. It is the least toxic of the bunch (ammonia, nitrite, and nitrate), but it can still be toxic – even if it is at high levels.