Glycogen is a polysaccharide consisting of glucose. It is a branched polymer — that means it is not linear/straight but has long chains of glucose coming off the sides of another glucose chain.

Animal (and human) cells store glucose in the form of glycogen. When glucose levels in the blood rise (e.g. after eating), insulin is released. Glucose is then taken up by muscle and liver cells, and converted into glycogen granules. This process is called glycogenesis.

When your body needs energy, or when blood sugar levels are low (between meals/fasting), glycogen is broken back down into glucose for use.