The term "sugar" is confusing. It refers to the stuff made from sugar cane but it also refers to any carbohydrate monomer (any single link in a carbohydrate chain). As we have noted elsewhere, when it comes to body weight, sugars are not equal. Unlike fructose, glucose has not been associated with the rebound hunger and leptin resistance seen with fructose. In other words, glucose, though it does have calories, is not an appetite stimulant like fructose and is therefore a generally better sugar for non-diabetic people.
On the other hand, if you DO have diabetes then glucose, like say, potato is something best avoided.
Glucose (also called "dextrose" or "blood sugar")' is the building block of starches and is released by the breakdown of those starches in the mouth (by salivary amylase) and in the gut (by pancreatic amylase). Interesting, pure glucose is not common in the American diet. Sources of pure glucose include 'old fashioned' corn syrup (think Karo) and 'dextrose' (the common name for glucose). The main reason glucose is not more commonly found in American foods is that it is not very sweet compared to other sugars.
Glucose is a major source of energy in the human body. Nearly every cell in the body can absorb and use glucose and some cells, like brain cells, can only use glucose (or, under starvation, chemicals called "ketone bodies"). Because it is so easily used, in normal, non-diabetic people, blood levels of it never rise too far before it is absorbed (through the action of the hormone called "insulin" (see below).
Diabetes is a disease characterized by excessively high blood glucose levels. Type 1 diabetes (childhood diabetes) happens when the pancreas loses most or all of its ability to make the hormone called insulin. Insulin is the main "trigger" to tell cells to absorb glucose. Type 2 diabetes (adult onset) is a different disease where the pancreas may make normal amounts of insulin but the cells that normally respond to insulin become resistant and respond less, absorbing less glucose from the blood and causing blood levels to rise. Simply put, type 1 diabetes is caused by too little insulin and type 2 diabetes is caused by too little reaction to insulin.
Glycemic index is a rough measure of how glucose-containing foods affect blood glucose levels. It was developed in Australia in the 1980s to help diabetic patients make food choices that better control their blood glucose levels. For diabetic people, glycemic index is an extremely valuable tool and from it we have learned that diabetics must limit not only sugar intake but also many starchy foods like potato and white bread.
Some advocates of glycemic index claim that it is a useful tool for weight loss. They argue that people who choose "low-glycemic" foods will find weight loss easier and more sustainable. This is where advice like "stay away from white starches" comes. As I will explain elsewhere, I do not have much faith in glycemic index for weight loss. Most of the weight that people do lose on a low glycemic diet is probably related to the fact that low glycemic foods have a lot of fiber (something we have long-known helps with weight loss) and not because of the glycemic index itself.
Plants store glucose as starch. Humans store it as molecule called "glycogen". Glycogen is found in liver and muscle and is sort-of like starch. Glycogen can be broken down into burnable glucose on demand through exercise or starvation.
Glucose is very hard for the body to make if we don't eat carbohydrate. Under carbohydrate starvation (think Atkins diet), the body will use up its glycogen within a day or so. Once the glycogen is gone, the body quickly switches to a different fuel called "ketone bodies". Ketone bodies can be made from the breakdown of fat. Glucose cannot (not well anyway). Normally we do NOT make ketone bodies but when our glucose stores run out we do. Ketosis (the condition of burning ketone bodies) causes bad breath and prevents intense exercise. We can survive in ketosis but it's unpleasant.
Provided you do not have diabetes, glucose may be a better choice because it doesn't cause the metabolic problems seen with fructose (found in table sugar, high fructose corn syrup and fruit juices. Think of it this way: eating glucose affects the body in precisely the same manner than eating a starchy potato does. If, as I have argued, starch is not a problem for non-diabetics, then glucose cannot be either. To put it another way, glycemic index is not a valid tool for weight loss.