Animal cells store excess energy as molecules of glycogen.
Animals (including humans) store some glucose in the cells so that it is available for quick shots of energy.
Excess glucose is stored in the liver as the large compound called glycogen. Glycogen, sometimes called "animal starch" for its resemblance with starch found in plants, is a polysaccharide of glucose, but its structure allows it to pack compactly, so more of it can be stored in cells for later use.
If you consume so many extra carbohydrates that your body stores more and more glucose, all your glycogen may be compactly structured, but you no longer will be.
Glycogenesis is the formation of glycogen from glucose. Glycogen is synthesized depending on the demand for glucose and ATP (energy). If both are present in relatively high amounts, then the excess of insulin promotes the glucose conversion into glycogen for storage in liver and muscle cells.