Fermentation is an anaerobic process for breaking glucose down into energy, and is an alternative to cellular respiration when oxygen isn't present to accept electrons at the end of the electron transfer chain. Fermentation and cellular respiration begin the same way, with glycolysis.
In fermentation, the pyruvate made during glycolysis does not continue on to the Krebs cycle and oxidative phosphorylation. Since oxygen isn't available, NADH can not drop its electrons to turn back into NAD+. The purpose of the fermentation reactions that follow glycolysis is to regenerate the electron carrier NAD+ from the NADH that was produced during glycolysis.
There are multiple fermentation pathways available, but in the case of alcohol fermentation, NADH donates its electrons to a derivative of pyruvate. This process produces ethanol.
Going from pyruvate to ethanol is a two step process. In step one, a carboxyl group is removed from pyruvate and released as carbon dioxide. This leaves acetaldehyde, a two carbon molecule. In step two, NADH passes its electrons to acetaldehyde. The NADH and acetaldehyde become NAD+ and ethanol.
Image from khan academy.org Fermentation and Anaerobic Respiration.