Aerobic - The second energy system, the aerobic system, kicks in when your short term, initial energy from the anaerobic system is used up. Aerobic exercise requires oxygen to produce energy.
An exercise is determined to be aerobic vs anaerobic depending on which system is used to generate the energy needed for a particular exercise.
Using running as an example, sprints are anaerobic and longer runs are aerobic exercise. When you sprint, you need explosive, quick energy to get going, so you use the anaerobic system. Readily available stores of ATP, glucose and glycogen provide energy quickly, but also deplete quickly. As these stores of ATP, glucose and glycogen deplete, you need a different system to produce ongoing energy.
As you run longer distances, you generally lower the intensity so that you can keep going. You need endurance and a system that produces energy as long as you need it. The aerobic system uses oxygen and produces ongoing energy needed for endurance activities, exercises and sports.
To exercise aerobically, you generally workout within a target exercise heart rate of between 60 to 85% of your maximum heart rate. Aerobic exercise trains your heart and lungs to become more efficient. Aerobic exercise is sometimes referred to as cardio. Examples of aerobic exercises are walking, running, cycling, hiking, swimming, water aerobics, aerobic dance, singles tennis etc.
To exercise anaerobically, you generally work at a higher intensity for a shorter durations or intervals. Examples of anaerobic exercise are sprinting, strength training, and doubles tennis.