These are organic molecules made up of carbon hydrogen and oxygen only in the ratio 1:2:1 via the formula (CH2O)n where n must be a whole number greater than or equal to 3.

We can thence conclude that the simplest sugar or carbohydrate is a 3-carbon sugar (triglyceraldehyde).

Sugars can be classified according the number of subunits they have into monosaccharides, disaccharides, oligosaccharides and polysaccharides. They can also be classified according to the type of carbonyl group they carry: aldosugars and ketosugars