What is insuline

It is a hormone, produced in the pancreas, that enables the body to metabolize Glucose, move it into the cells and turn it into energy. lack of insulin causes diabetes.

Here is some

It's a hormone which helps regulate sugar level in the blood for various uses. People with diabetes 1 suffer from insufficient insulin, and people with diabetes type 3 suffer from insulin resistance, which means usually that they have too much sugar in their blood.

It is also responsible for gaining body mass (not fat). 

I agree with the answers posted but I think that Slayerjack meant diabetes type 2 and not 3 (unless there is a type I don't know about). I just wanted to add that today researchers are discovering other important functions that the Insulin has on other areas of the body. It turns out that the story doesn't end with helping the glucose enter cells.

i would agree with "Angelchelsea", and would add that "lack of insulin" is not the only cause of diabetes.

but as your question is insulin specific: 

insulin is a hormone (and as such, a protein.) that is indeed provided by the pancreas, it is secreted by a small group of cells, within the pancreas, called the islet cells. the purpose of insulin is indeed to regulate the release of energy, from the food that we eat, into the cells that require it.

hope that this helps. 

This is from wikipedia and if you want you can read the whole wiki here

Insulin is an animal hormone whose presence informs the body's cells that the animal is well fed, causing liver and muscle cells to take in glucose and store it in the form of glycogen, and causing fat cells to take in blood lipids and turn them into triglycerides. In addition it has several other anabolic effects throughout the body.

Insulin is used medically to treat some forms of diabetes mellitus. Patients with type 1 diabetes mellitus depend on external insulin (most commonly injected subcutaneously) for their survival because of the absence of the hormone. Patients with type 2 diabetes mellitus have insulin resistance, relatively low insulin production, or both; some type 2 diabetics eventually require insulin when other medications become insufficient in controlling blood glucose levels.

Insulin is a peptide hormone composed of 51 amino acid residues and has a molecular weight of 5808 Da. It is produced in the Islets of Langerhans in the pancreas. The name comes from the Latin insula for "island".

 

Hope that helps