The monomers of carbohydrates are known as monosaccharides. The Royal Society of Chemistry describes monomers as the most basic building blocks of carbohydrates. They are required to make more complex carbohydrates, such as disaccharides and polysaccharides.
Monosaccharides share a common chemical formula, (CH2O)n, in which "n" represents the number of carbon atoms within a particular monosaccharide. Monosaccharides with two to seven carbon atoms are common, but Wikipedia describes those with eight or more carbons as very rare due to their instability.
Glucose, galactose, fructose, ribose and deoxyribose are among the most recognizable monosaccharides. Wikipedia states that glucose is particularly important because it is the fuel source required by the cells of nearly all known organisms. Ribose and deoxyribose are also important because of their use in the creation of RNA and DNA.
According to the Royal Society of Chemistry, monosaccharides are rare in nature in comparison to other types of carbohydrates. Carbohydrates are more commonly found as disaccharides or polysaccharides.
Wikipedia states that chains of monosaccharides are held together by glycosidic bonds, which are unique to carbohydrates. Disaccharides and polysaccharides are generally only used as a form of storage. To make them available for cellular use, the glycosidic bonds must be cleaved during metabolism and they are split back into monosaccharides.