In aerobic respiration energy is released during the oxidation process involving oxygen.
In anaerobic respiration energy is released during the oxidation process involving oxidizing agents such as sulphates SO4, Sulfur S, nitrates NO3 etc. The chemical reactions give out energy but less compared to aerobic reactions.
Energy is produced during the process called glycolysis like in aerobic respiration. In this respiration, ATP is synthesized from (ADP) Adenosine diphosphate and inorganic phosphates. Energy is released also from Krebb's cycle by oxidation of acetates from carbohydrates.
Many prokaryotes (anaerobic organisms) respire only using anaerobic respiration.