The mono's are the polyhydroxy aldehydes and ketones and cannot be hydrolyzed to simpler sugars. For this reason they are sometimes referred to simple sugars. Sugars are also classified as either aldoses or ketoses. Combining the two, ribose is a aldopentose and fructose is a ketohexose. Some of these may not be polyhydroxy aldehydes or ketones, but they yield them upon hydrolysis so they meet the definition of CHO's.
Polysaccharides are many simple sugars bonded together.
They are subclassified as either homopolysaccharides or heteropolysaccharides depending on whether or not the sugars in the polymer are all the same or are different. Starch is a polymer of just glc. In the Fisher projection of D-glyceraldehyde note the chiral center. Molecules that have the same empirical and structural formula, but differ only in the orientation of groups bound to the chiral carbon are related as stereoisomers. The mirror image can be represented and this is the L form.