The human body is like an automobile. We have a fuel tank that takes in and holds fuel, an engine that converts fuel to allow the vehicle to function, an interconnected piping and wiring system to move the energy throughout, and wheels to permit locomotion.
Our fuel is the food we eat: carbohydrates, proteins, and fats. In order to properly utilize these macronutrients, we exploit the chemical component in food and convert it to ATP (adenosine triphosphate). ATP is an intermediate our body uses as energy after food has been broken down. This is similar to a car pushing combusted fuel through its pipes to activate various parts of the car. ATP is important because it is the directly compatible form of energy in the body like combusted gasoline in a car. Gas without combustion would not provide energy in the car as food without metabolization would not provide ATP energy in the body.
We eat foodstuffs, our digestive tract breaks down this food into usable chemical components, our body mobilizes the energy in the blood and sends it throughout the body.
This is now where the different energy systems come into play.
Each of these forms ATP in a different way. The phosphocreatine (PCr) system utilizes substrate created by the body that is already stored in the muscles. Muscle cells build up their stores of PCr while at rest. Creatine phosphate provides energy for explosive bursts of activity up to 10 seconds: box jumps, snatch and clean weightlifting movements, and short sprints (up to 100meters). This system will always be used first to supply energy when you begin to start an activity.
Glycolytic metabolism is an intermediate source of energy. It metabolizes glucose from the blood or glycogen in the muscles and liver, these are derivatives of carbohydrates. Glycolytic metabolism provides energy to maintain all-out effort from 30seconds up to ~2 minutes at moderately-high intensity. These activities utilize both power and endurance such as middle distance running, rowing, cycling, and swimming, or high rep, high-intensity circuit training with short rest periods. If you continue exercise beyond two minutes, your body will feel crampy and acidic and you will be forced to slow your pace.
Oxidative metabolism provides energy for long duration low to moderate intensity activities. As indicated in the name, oxidative metabolism requires oxygen to get energy (ATP) from carbohydrate and fat. It does this by fueling off our bodies glucose, similar to glycolytic metabolism, however, it also pulls from our bodies fat stores. Oxidative metabolism provides ATP at a slower rate than other energy systems which is why it is the primary energy system for activities such as marathon running, long walks, bike rides, and even sleeping.
Baechle TR, & Earle RW. (Eds.). (2008). Essentials of Strength and Conditioning – third edition. National Strength and Conditioning Association. Champaign, IL: Human Kinetics.