Fermentation in biological systems is a metabolic process that converts sugar to acids, gases, or alcohol in the absence of oxygen. It begins with the breakdown of glucose through glycolysis, generating pyruvate, which is then converted into other products like ethanol or lactic acid, depending on the organism and conditions. Fermentation is widely used in various applications, including food production (such as in bread and yogurt) and biofuel generation, as it allows for anaerobic energy production.