Aerobic and anaerobic are antonyms to each other. Aerobic means "with oxygen", so aerobic exercise is a metabolic process that requires oxygen to run through. Anaerobic means "without oxygen". When muscle does not have enough oxygen during operation, the exercise will be anaerobic. For human body, aerobic exercise can increase endurance and improve heart health, while anarobic exercise can increase muscle strength. Basically, most aerobic exercises are of low intensity, while anarobic exercises are of high intensity. In other words, too much anarobic exercise will lead to overtaining, and most importantly it will make your body fell tired. In terms of biology, the amount of  lactic acid and hydrogen ion will increase and lead to exercise-induced fatigue and  lower ability of physical activity in a while. This is why anaerobic exercises burns more fat but makes people feel more tired.