Glycogen

Glycogen is the stored form of carbohydrate (specifically glucose) existing mainly in the muscles and liver. Humans form glycogen as a means of storing excess glucose. Glycogen is broken back down into glucose when energy is needed (a process called glycogenolysis).

Muscle glycogen is used to fuel physical activity. With prolonged activity, these glycogen stores are rapidly used up, and the body then relies on fat for fuel. Muscles with a plentiful supply of glycogen are able to exercise longer without tiring.

Liver glycogen is stored and released to help regulate blood glucose levels and to ensure a steady stream of glucose to the brain for normal functioning.

Glycogen partly consists of water. Each gram of glycogen is believed to draw 2.7g water into storage along with it. In times of activity this water is rapidly lost and must be replenished to help revive energy stores. In times of carbohydrate restriction in the diet, glycogen is also believed to be depleted and water lost. 

There is some evidence that intense exercise and a high-carbohydrate diet ("carb loading") can increase the reserves of glycogen in the muscles and thus may help endurance athletes work their muscles longer and harder than otherwise. But for non-athletic individuals, carb loading can lead to increased deposits of body fat, since stores in muscles and liver are likely to be filled.