Definition - What does Anaerobic Respiration mean?
Anaerobic respiration is a form of respiration that is contrasted with aerobic respiration in that it does not use oxygen. It is an alternative method of respiration that is used when oxygen is not present and derives energy from other chemicals nearby or within the body. Aerobic respiration produces more energy and is 15 times more efficient than anaerobic respiration. This is why most large creatures breathe to take in oxygen.
Safeopedia explains Anaerobic Respiration
Some smaller organisms, such as single celled organisms, do not require as much energy as others and can get by using anaerobic respiration. This method of respiration is not limited to these creatures. Larger animals, including humans, use this method as a backup when conditions do not allow for aerobic respiration to take place successfully, such as with extreme exertion, where oxygen alone may not be enough.