Through routine aerobic activity, the body becomes more efficient at processing oxygen. Examples of aerobic activity include running, jogging, biking, walking. 

In fact any exercise that incorporates large muscle groups, raises the heart rate, breathing rate and body temperature for a prolonged amount of time is aerobic in nature. 

Benefits

1.    Increases cardio respiratory and cardiovascular  system outputs
2.    Strengthens heart
3.    Decreases resting heart rate
4.    Improves circulation by clearing out cholesterol 
5.    Body adapts to burn fat as primary fuel source
6.    Improves psychological disposition and reduces 
        stress levels
7.    Raises basal metabolic rate
8.    Decreases blood pressure
9.    Reduces LDL blood cholesterol level
10. Tones muscles
11.  Improved balance and posture
12.  Increases Blood Oxygen level
13.  Increases flexibility, reducing capability for  injury

Weekly Requirements 
Fitness Level gains are determined by Frequency, Intensity and Duration of the Aerobic exercise. 

Each session  should last from 20 to 60 minutes 
Performed 3 to 5 days per week (frequency)

At a intensity level measured by heart rate 

(60% - 90%) 

During the first 15 minutes of aerobic activity, glycogen or sugar within the muscles is used for energy. Fat metabolism for energy won't occur until about 15 to 20 minutes after beginning aerobic activity. 

This is why it's important that 
aerobic duration be at least 30 minutes. Aerobic sessions greater than 1 hour continue to burn fat but at not the same rate as during the first hour. 

Additionally, sessions greater than 1 hour increase the risk of injury due to fatigue. After 60-90 minutes of continues aerobic activity the glycogen stores of energy are depleted.

Proper rest and sleep is critical because the body only becomes fitter when it recovers from the stress placed upon it during exercise.