Cellular respiration is the chemical reaction that happens continuously in every living cell, in plants as well as animals. It releases energy from glucose. It never switches off, because cells always need energy to stay alive.
The energy released is used for metabolic processes, which are all the chemical reactions in a cell. Examples are building large molecules such as proteins from smaller ones, making muscles contract, active transport and keeping the body warm.
Respiration is an exothermic reaction. This means it transfers energy to the surroundings, mostly as heat, which is why your body feels warm after exercise.
There are two types. Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water. Anaerobic respiration takes place without oxygen and releases less energy.