What Is Insulin?

Insulin is a hormone that facilitates glucose uptake by cells. It also facilitates storing of excess glucose in the liver as glycogen. Insulin also helps to prevent using stored fat as an energy source. All of these functions help us to maintain a constant blood glucose level.
Insulin is produced in the pancreas. It is then released into the blood if the glucose concentration is high in blood. This causes glucose to be removed from the blood by being taken up by the cells. Insulin also simultaneously prevents lipolysis (breakdown and use of stored fat). When blood glucose is low (when you haven't eaten in a while), the insulin level is also low, which facilitates the use of your stored fat stores for energy.
When the body loses its ability to react to the presence of insulin, the person is said to be suffering from Diabetes mellitus type II. The insulin is being produces in the body but the cells do not respond to it and hence, do not take up glucose.
There is a very good movie that I found on Youtube. It shows the release of insulin and how it causes cells to take up glucose. It also shows what happens during Diabetes mellitus type II:

Insulin