Glucose, a simple sugar monosaccharide, is one of the most important carbohydrates and is used for energy in plants and animals. The natural form (D-glucose) is also referred to as dextrose, especially in the food industry.
Glucose (C6H12O6) is a hexose--a monosaccharide containing six carbon atoms. Glucose is an aldehyde (contains a -CHO group). Five of the carbons plus an oxygen atom form a loop called a "pyranose ring", the most stable form for six-carbon aldoses. In this ring, each carbon is linked to hydroxyl and hydrogen side groups with the exception of the fifth atom, which links to a 6th carbon atom outside the ring, forming a CH2OH group.
There are two enantiomers (mirror-image isomers) of the sugar -- D-glucose and L-glucose. Yet in living organisma only the D-isomer is found. The ring structure may form in two different ways, yielding alpha-glucose and beta-glucose. Structurally, they differ in the orientation of the hydroxyl group linked to the first carbon in the ring. The alpha form has the hydroxyl group "below" the hydrogen (as the molecule is conventionally drawn, as in the figure above), while the beta form has the hydroxyl group "above" the hydrogen. These two forms interconvert on a timescale of hours in aqueous solution.
The older name dextrose arose because a solution of D-glucose rotates polarised light towards the right. In the same vein D-fructose was called "levulose" because a solution of levulose rotates polarised light to the left.