Remember, under aerobic conditions, we have our citric acid cycle and we have the electron transport chain that actually produces the majority of the ATP molecules, as well as oxidizes the NADH back into the NAD plus. But under anaerobic conditions, we have glycolysis being the only method by which we produce our ATP molecules. And to regenerate those NAD plus, we have to undergo the process of fermentation. Now, whenever somebody says fermentation, that basically incorporates glycolysis. So glycolysis followed by this process or this process together is known as fermentation. So fermentation produces a net of two ATP molecules when we take one glucose and break it down, so we see that these processes do not actually themselves produce any ATP molecules.