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In a Fischer projection, each cross represents a carbon position: horizontal chemical bonds point toward the viewer and vertical chemical bonds point away. For the open-chain aldehyde sugars illustrated here, place $\mathrm{CHO}$ at the top and ${\mathrm{CH}}_2\mathrm{OH}$ at the bottom. This is a convention for stereochemistry, not a measured molecular shape. See Moss, IUPAC Recommendations 1996: Fischer projection.

In these Fischer projections, D specifies that the hydroxy group at the lowest stereocenter lies on the right: C4 for ribose and deoxyribose, C5 for glucose. D does not specify every other stereocenter, and it does not specify the direction of rotation of polarized light. See McMurry: the meaning of D and L.

Stereochemistry studies the spatial arrangement of chemical atoms in molecules. A Fischer projection records this information at successive stereocenters, whereas a molecular formula only counts chemical atoms. See McMurry: representing carbohydrate stereochemistry.

In chemical usage, sugars commonly means small carbohydrates, including single-unit monosaccharides and two-unit disaccharides. Glucose, ribose, and deoxyribose are monosaccharides. Cellulose is instead a polysaccharide assembled from many glucose units. The distinction counts chemically linked units; it does not classify every carbohydrate by sweetness or biological role. See Clark, Douglas and Choi, Biology 2e, 3.2: carbohydrates.

A useful first description of a monosaccharide asks how many carbon chemical atoms it contains and what its open-chain structure is. Ribose is an aldopentose, a five-carbon aldehyde sugar; glucose is an aldohexose, with six carbon chemical atoms. A Fischer projection then records the stereochemistry, information missing from the molecular formula. See McMurry, Organic Chemistry, 25.1: carbohydrate classification.