Fermentation is one of the most common pathways for anaerobic cellular respiration to occur, cellular respiration is the process in which a fuel, usually glucose, is processed through an electron transport chain to produce ATP. Fermentation is similar to many other cellular respiration pathways in the fact that it begins with glycolysis producing NADH and pyruvate. Due to a lack of oxygen, the electron transport chain in unable to function inhibiting the NADH to revert back into NAD⁺, without NAD⁺ the cycle of glycolysis cannot repeat. Additionally, pyruvate is unable to proceed through oxidization and the citric acid cycle, because of this one or two additional processes that are capable to occur in an anaerobic environment are then added. These processes are similar in the fact that they allow the NADH to drop of its electrons onto the organic molecule pyruvate or a molecule formed from the pyruvate. As there are many fermentation processes for many unique situations, the two most commonly known are lactic acid fermentation and alcohol fermentation.  Lactic acid fermentation is carried out by muscle cells when the necessary oxygen needed or the cells normal aerobic processes is absent, usually during exercise. Alcohol fermentation is used to produce the ethanol in alcoholic beverages, the NADH donates its electron into a molecule called acetaldehyde, which intern creates the ethanol. This occurs in a two-step process in which a carboxyl group is removed from the pyruvate, producing the two molecules of acetaldehyde, later the NADH donates its electron onto acetaldehyde, regenerating NAD⁺ and producing ethanol. Fermentation is not efficient in producing ATP, but without it producing ATP in abnormal conditions with a lack of oxygen would be impossible.

References:

https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/variations-on-cellular-respiration/a/fermentation-and-anaerobic-respiration