The words aerobic and anaerobic refer to the way energy is produced during exercise. Aerobic means "with oxygen" and anaerobic means "without oxygen".
Carbohydrates are broken down more efficiently aerobically than anaerobically.
Fatty acids are the preferred source of energy during low intensity aerobic activities (e.g jogging). This allows muscle cells to be supplied with enough oxygen during exercise to continue working aerobically while using fat as the main energy source.
During aerobic exercise such as running, cycling and swimming (which last longer than 2-3minutes) there is more time for oxygen to be delivered in the blood to the muscles and so aerobic production of energy can occur.