I met with the following question in my masters course:
What do different fermentation products have in common?
Thinking about this, I have difficulty in identifying the similarities between lactic acid and alcohol fermentation.
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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.
Q1. What do different fermentation products have in common?
A1. They are generated in a reaction in which NADH is reoxidized to NAD+, allowing anaerobic oxidation (e.g. of glucose) to continue.
Q2. What are the similarities between lactic acid and alcohol fermentation?
A2. They both generate ATP ‘at the substrate level’ (i.e. directly in reactions, without using oxidative phosphorylation) in a sequence of reactions which involve oxidation by NADH. The end-product of the oxidation (here pyruvate) is of no further use for anaerobic generation of ATP and so it is unimportant what it is reduced to by NADH.
This is illustrated below (see also this question):
(As mentioned in the answer from @xusr, pyruvate is converted to acetaldehyde before the NADH step.)
There are other types of fermentation with different end-products — see e.g. the Wikipedia article on fermentation — but the general principle is the same.