To put it very simply, diabetes mellitus is a chronic and currently incurable condition in which too much glucose (sugar) is present in the blood.

Why should this be so?
Since diabetes is a disease that affects your body's ability to use glucose, let's start by looking at what glucose is and how your body controls it.

Glucose is a simple sugar, which is normally necessary to provide energy to the cells in your body. The various cells in your body take up the glucose which is in the blood and break it down using various biochemical pathways into "energy". Whilst in the absence of glucose, many cells and organs can use other substitutes for a while, important cells such as those of the brain, the red blood cells and the those of kidneys can only use glucose and nothing else.

When you eat food, the carbohydrates in the food, and remember most of the diet is full of carbohydrates (starch, in popular parlance), gets broken down in the intestines into simple sugars such as glucose which are then absorbed into the bloodstream and get distributed all over the body for it to be used. When you have something with simple sugars, the glucose gets absorbed that much faster!

But we do not eat continuosly. So our body has a mechanism which closely regulates the level of glucose in the bloodstream so that the cells of the body can get their required supply at all times. Unless this is done, your cells would have too much glucose right after a meal and starve in between meals and overnight !

So, when you have an oversupply of glucose, your body stores the excess in the liver and muscles by making glycogen, long chains of glucose. When glucose is in short supply, your body mobilizes glucose from stored glycogen and/or stimulates you to eat food.

The key is to maintain a constant blood-glucose level. Think of this as some sort of a "depot" where you store excess materials for later use. So how does the body fine tune this mechanism by which a normal amount of glucose is available for use by the body at all times?

To maintain a constant blood-glucose level, your body relies on two hormones produced in the pancreas that have opposite actions: insulin and glucagon. Although things are not that simple and many other mechanisms do come into play, it will suffice for our purposes to understand the basic role played by these two hormones.