Your body needs energy to function. However, your body being amazing was not satisfied with just creating a single type of energy for itself and went ahead and created three. To cater to whatever catches your fancy at a given point as it were. These metabolic energy pathways or systems are called phosphagen system, glycolysis and anaerobic. Each caters to a different energy need and uses different energy sources as fuel. The energy we require in our body is created through a process of breaking down Adenosine Tri Phosphate (ATP) into lower energy phosphates called Adenosine Di Phosphate. This is a hydraulic process which means it requires water.
If you fancy yourself as a weight lifter, someone who can sweep me off my feet, no I kid, someone who can lift say two hundred pounds, then your body will use its phosphagen system to power the energy requirement. No carbohydrate or fat is used in this process and instead the body consumes creatine phosphate for its energy.
If your spend a lot of time in the gym and need energy to power short bursts of activity from say thirty seconds to even two minutes, lifting the kettle bell and squatting for example, your body will use the glycolysis system. In this energy system your body uses blood glucose or sugar or muscle glycogen to fuel its energy needs. As an outcome of glycolysis your body can produce lactate or acetyl coenzyme A (acetyl-CoA). Lactate is created when your body is oxygen deficit which makes you feel the burning sensation in your body when you are working out.
If you are a long distance runner and need ongoing energy to fuel your desire to touch the tape at the end of the 42 Km mark, you need to rely on your anaerobic system to fuel your energy needs. This is a far more complex energy system and is dependent on oxygen. In this energy system your body uses blood glucose, glycogen and fat to fuel itself. If your body runs out of its required fuel source, it can turn to muscle to feed its energy requirement an important reason that you need to both hydrate yourself as well as eat well before any long distance run.