Diabetes

In human body there is a hormone called Insulin. Its work is to make our body cells take in glucose from the blood. The glucose is accumulated in the liver and muscle as glycogen and stops the body from using fat as a source of energy.
The problem comes when there is very little insulin in the blood, or none at all; glucose is not taken up by most body cells. 
When this happens our body uses fat as a source of energy. Insulin is also regarded a control signal to other body systems, such as amino acid uptake by body cells. 

Diabetes is defined as a metabolism disorder. Metabolism means the way our bodies use digested food for energy and growth. Most of what we eat is turned into glucose. Glucose is a form of sugar in the blood and it is the principal source of energy for our bodies. 
When our food is digested the glucose makes its way into our bloodstream, our cells use the glucose for energy and growth. However, glucose cannot enter our cells without insulin being present - insulin makes it possible for our cells to take in the glucose. Insulin is a hormone that is produced by the pancreas. 

When we eat something the pancreas automatically releases an adequate quantity of insulin to move the glucose present in our blood into the cells, and lowers the blood sugar level.
A person suffering with diabetes faces a condition in which the quantity of glucose in the blood is too elevated. The reason behind this is that the body does not produce either enough insulin, produces no insulin, or has cells that do not respond properly to the insulin the pancreas produces.
This results in too much glucose building up in the blood. This excess blood glucose eventually passes out of the body in urine. So, even though the blood has plenty of glucose, the cells are not getting it for the necessary energy and growth requirements according to the body. And that is all called diabetes. 

Insulin is not identical in all animals - their levels of strength are different. Porcine insulin, insulin from a pig, is the most similar to human insulin.
Humans can receive animal insulin. Still, genetic engineering has permitted us to synthetically produce human insulin rather then to get it from any other source.