What do different fermentation products have in common?
Although there is  no net oxidation or reduction in glycolysis, a constant supply of  NAD+ is (paradoxically) required in order for glycolysis to continue. Otherwise, the glyceraldyde-3-phosphate dehydrogenase reaction (GAPdh) could not generate 1,3-diphosphoglycerate. 

So how is the NADH generated in the GAPdh reaction converted back to NAD+? Both alcohol and lactic acid fermentation regenerate NAD+ by reduction of the carbon skeleton that was oxidized in the GAPdh reaction, albeit at different 'points' in the glycolytic reaction sequence.  

In alcohol fermentation, alcohol dehydrogenase acts as aldehyde reductase, reducing acetaldehyde to ethanol and NAD+.  In lactic acid fermentation, lactate dehydrogenase acts as a pyruvate reductase, reducing pyruvate to lactate and NAD+.

In both alcohol and lactic acid fermentation, there is no net oxidation or redution: glycolyis is the splitting of glucose. 

Finally, we may note that the electron acceptor in both cases is 'internal'  Neither process requires an external electron acceptor such as oxygen.  The electrons removed from the carbon skeleton in the GAPdh reaction (generating NADH) are added back to the same carbon skeleton (but with different molecular arrangement) in both the ADH and LDH reactions.