Aerobic exercise is exercise that increases the heart rate but still relies on aerobic metabolism.
A good rule of thumb to know if you are in the aerobic zone is that you can carry on a conversation while you exercise, but you cannot sing.
Aerobic training increases overall endurance by causing physiological changes to the cardiovascular system.
Aerobic exercise strengthens the heart muscle, allowing it to be more efficient.
It increases the amount of blood that can be pumped with one beat of your heart.
As this increases, your resting heart rate decreases because it takes fewer beats to pump the same amount of blood.
A typical resting heart rate is between 65 - 100 beats per minute.
Well-trained athletes can have resting heart rates of 35-40 beats per minute.
The lowest recorded heart rate was 28 beats per minute, recorded on Spanish cyclist, Miguel Indurain.
Aerobic exercise also causes a number of other changes that increase your aerobic capacity:

• Increases the number of capillaries feeding your muscles so that blood is delivered more efficiently
• Increases the number of red blood cells and the total volume of blood, thus increasing the total amount of hemoglobin greater, allowing more oxygen to be delivered
• At the cellular level, improves the ability of muscle cells to use aerobic metabolism
• Increases both the number and size of mitochondria in the muscle cells,
• boosts the activity of enzymes within the mitochondria that are needed for aerobic metabolism and fatty acid breakdown
• Increases the availability and transport of fatty acids to the mitochondria
• increases triglyceride storage and oxidation in the muscle
• Builds up the muscle cells capacity to burn fatty acids to produce ATP (Adenosite Tri Phosphate).
  Using fatty acids instead of glycogen delays the onset of muscle fatigue
• Increases the ability to store glycogen

Trained athletes store more glycogen and use it more slowly; thus, they can exercise for longer periods at higher intensity than an untrained person can.
A conditioned person can also operate at a higher intensity before lactic acid begins to develop.