Carbohydrates are carbon-based macromolecules based on C, H and O. They are polar and readily dissolve in water.
A simple sugar by itself, like glucose, is termed a monosaccharide, and serves as a 'building block' (monomer) to build a larger chain of many molecules called a polymer. In the case of carbohydrates, the polymer made from many monosaccharides linked together by dehydration reactions is called a polysaccharide.
Examples of polysaccharides made of glucose include cellulose, glycogen and starch. Each of these glucose polymers are biologically important.