Cellular respiration is the chemical process that releases energy from food molecules. It happens in every living cell, in plants, fungi, animals and microorganisms, and it carries on all the time while the organism is alive. This is why it is called a universal process that is continuously occurring.
Respiration is not the same as breathing (ventilation). Breathing moves gases in and out of the body, whereas respiration is the chemical reaction that happens inside cells.
The energy is not used straight away. It is used to make a molecule called ATP. ATP carries energy around the cell and releases it where it is needed, for example for muscle contraction, active transport and building large molecules. Organic molecules such as glucose are the fuel. The word equation is: glucose + oxygen → carbon dioxide + water.