Fermentation is a way to obtain energy from carbohydrates when there is no oxygen available. Fermentation can be realized for many organisms, even when is not the main source of energy. For example, humans can realize lactic fermentation.
There are different types of fermentation depending on what substance is produced. In ethanol or alcoholic fermentation, the organisms take the carbohydrates and produce ATP, ethanol, and carbon dioxide. An example of organisms that perform alcoholic fermentation is yeast.