Dextrose, fructose, and glucose are all monosaccharides, known as simple sugars. These sugars can combine to form more complex sugars, including the disaccharide known as sucrose, or table sugar. The primary differences between these sugars have to do with the ways in which they are metabolized by the body, and they all play important roles in body function.
Fructose is an extremely sweet sugar that is found in many fruits. The flavor is sometimes intensely cloying, as anyone who has eaten an overripe piece of fruit knows, and it is believed to be the most sweet of the naturally occurring sugars. Fructose can also be obtained through the breakdown of sucrose, which is made from linked fructose and glucose molecules. It has a low glycemic index, which means that it takes a long time for the body to break down, resulting in a slow release of sugar, rather than a sudden rush. A diet high in fructose as been linked to health problems like diabetes and obesity, although there is as of yet no proof that it causes these conditions.
Dextrose is simply a form of glucose. Some food packagers like to use "dextrose" on their packaging instead of "glucose" because they believe that people have negative associations with glucose. This sugar is extremely abundant in nature, and it can be found in numerous plant and animal tissues, often along with other sugars such as fructose. The body relies on glucose for energy, using this sugar to power cells. When people measure their blood sugar, they are actually measuring the amount of dissolved glucose in the blood.
The molecular formula for glucose/dextrose and fructose is actually the same. Both sugars are considered to be hexoses, meaning that they have six carbon atoms attached to 12 hydrogen atoms and six oxygen atoms. The differing ways in which molecules can be attached cause various hexoses to behave differently, creating different chemical compounds which lend the hexoses some distinct properties.
Glucose is what is known as an aldohexose, meaning that it contains a compound called an aldehyde, located at the first position in the molecule. Aldehydes have a carbon atom which is attached to a hydrogen atom and also double bonded to an oxygen atom. Fructose, on the other hand, is a ketohexose, containing a ketone which consists of a single carbon atom double bonded to an oxygen atom. The ketone in fructose is attached to the second position in the molecule. Ketones play an important role in biochemistry.
The simple structures of these sugars allow them to be linked in a number of different ways to other molecules, creating more complex sugars which will behave differently in the body and generating some extremely unwieldy chemical formulas. For consumers, the important thing to remember is that labels which say "dextrose" really mean "glucose."