Respiration refers to the process of converting food substances such as glucose into energy which the cell can use.
There are two types of respiration - aerobic and anaerobic respiration. Both these processes occur in the mitochondria within cells.
Aerobic respiration occurs when there is oxygen present. It can be summarised by the equation: glucose + oxygen --> carbon dioxide + water (+ energy). Aerobic respiration is present in animal and plant cells.
Anaerobic respiration occurs when there is little to no oxygen present and results in the incomplete breakdown of gucose. It is far less effective than aerobic respiration and can be summarised by the equation: glucose --> lactic acid (+ little energy).