Diabetes is a chronic and currently incurable condition in which too much glucose (sugar) is present in the blood.
In a nut shell, the problem with people suffering from diabetes is that they cannot utilize the glucose in their blood. Glucose is a simple sugar and the body cells use this glucose and convert it into energy. A hormone called Insulin is the key which opens the door that lets glucose into the cells so that the cells can use them to make fuel. In Diabetes the hormone Insulin is either produced insufficiently, or the body resists the action of Insulin on cells. So the glucose builds up in the blood, and it goes waste as the glucose cannot reach inside the cells.