Anaerobic cellular respiration generates ATP when insufficient oxygen is available for aerobic respiration.
Anaerobic respiration takes place after glycolysis. This process differs among species, and in mammals it is called lactic acid fermentation. Lactic acid fermentation does not need oxygen to proceed, although it can occur in its presence. During lactic acid fermentation, pyruvate is converted into lactic acid and the NADH produced during glycolysis is recycled. The net release of energy comes from glycolysis in the form of 2 ATP molecules.
Overview of anaerobic cellular respiration in mammals.