Sugar is what gives us energy. Our bodies convert the carbohydrates we eat into sugar, which they send via the blood stream to every cell in the body.
For the cells to get the sugar from the blood they need to open a gate (so to speak). To do that, they use the services of a gatekeeper, i.e. insulin.
If the body does not have enough insulin, the sugar cannot be transmitted from the blood to individual cells, hence we do not have enough energy, and have all slew of other problems. This condition is called diabetes.
The state of having too much blood sugar (whether due to diabetes, or temporarily for eating too much sugar) is called hyperglycemia.
If we have too little blood sugar, the cells do not get enough energy either. This is called hypoglycemia.
Both hypoglycemia and hyperglycemia can cause a coma and, if not treated, result in death.
Blood sugar can be measured quite easily with a glucometer, which only needs a drop of blood and about 45 seconds of time.
I have diabetes, so I have to measure my own blood sugar frequently. Every doctor and nurse tells me slightly different values of what is good.
But on the label of the vial that contains my test strips, the following control ranges are listed:
HIGH LOW NORMAL
264-337 41-58 84-126 MG/DL
14.7-18.7 2.3-3.2 4.7-7.0 MMDL/L
The glucometer I use gives me the results in mg/dl, that is the first line above. Anything below 150 makes me happy, anything in two digits makes me ecstatic.