Carbohydrate

A carbohydrate is an organic compound that is made up of carbon, hydrogen and oxygen and is considered as one of the major class of biomolecules.The general formula of carbohydrate is C_n(H₂O)_n. One of the most commonly known carbohydrate would be glucose which is one of the main energy source for living organisms.
Monosaccharides are the simplest carbohydrates such as glucose and galactose. They are generally aldehydes or ketones that have multiple hydroxyl groups attached. The smallest monosaccharides are dihydroxyacetone and D- and L-glyceraldehyde.  Dihydroxyacetone is a ketose because it contains a keto group, whereas glyceraldehyde is an aldose because it contains an aldehyde group  (there are two stereoisomers of glyceraldehyde due to a single asymmetric carbon atom).

References

Berg J., Tymoczko J and Stryer L. (2001) Biochemistry fifth edition, New York:WH Freeman. pg 301

When simple monosaccharides (such as maltose) are linked together through 1,4-glycosidic bonds to other monosaccharides, a wide variety of information rich oligosaccharides are formed that can be used in complex processes such as cell signalling.