Respiration happens continuously in every living cell. It is an exothermic reaction, so it transfers energy to the surroundings. That energy supplies everything the organism needs to stay alive: building larger molecules from smaller ones, movement, and keeping warm.
Cells can respire in two ways. Aerobic respiration uses oxygen. Anaerobic respiration happens without oxygen. The aerobic word equation is glucose + oxygen → carbon dioxide + water, which in symbols is C6H12O6 + 6O2 → 6CO2 + 6H2O.
In animal muscles, anaerobic respiration turns glucose into lactic acid only: glucose → lactic acid. Glucose is only partly oxidised, so much less energy is transferred than in aerobic respiration, where it is fully oxidised.
Plant and yeast cells respire anaerobically in a different way: glucose → ethanol + carbon dioxide. In yeast this is called fermentation. It matters economically because the carbon dioxide makes bread dough rise, and the ethanol is the alcohol in alcoholic drinks.