Combustion is burning something in presence of abundant oxygen
Respiration is the breaking down of food using oxygen to produce energy
Combustion is spontaneous oxidation of a substance in abundant oxygen. It is an uncontrolled process. It involves no enzymes and the energy released is in large amounts.
Respiration is a bio chemical redox reaction in which the food is broken down into simpler substances to produce energy in the presence of oxygen . It involves enzymes and is a controlled process. The energy released is in the form of ATP and is not used instantly but stored.