Carbohydrates are among the most abundant substances on the Earth and perform many vital functions in both plants and animals. The most common of these are cellulose, starch, and various sugars. Cellulose is the main structural component of plants, used to construct rigid cell walls, fibres, and woody tissue. Starch is the chief form for storing carbohydrates for later use as a food or energy source. In higher animals, the sugar glucose is an essential component of blood. Ribose and 2-deoxyribose are constitiuents of genetic material. Other carbohydrates are important components of coenzymes, antibiotics, cartilage, the shells of crustaceans, and bacteria cell walls. For the last decades a definite role played by carbohydrates in the processes of cell recognition and immunity was established.

Carbohydrates being the initial products of photosynthesis (from carbon dioxide and water) represent a certain «bridge» between inorganic and organic substances.

The term carbohydrates, which has been used since the mid-nineteen century, derives historically from the observation that many compounds in this class (for example ribose, C₅H₁₀O₅, glucose, C₆H₁₂O₆, and sucrose, C₁₂H₂₂O₁₁) have the empirical formula C_x(H₂O)_y and are formally regarded as «hydrates of carbon». However many important compounds related to carbohydrates, such as deoxyribose, C₅H₁₀O₄, uronic acids, and amino sugars (see further in this chapter), are not covered by this definition. Nevertheless, this term is steadily employed along with the equivalent names saccharides or simply sugars.

It is difficult enough to give a clear definition of carbohydrates, because they include very different types of compounds, from small molecules with several carbon atoms to polymers whose molecular mass amounts to millions. Sometimes carbohydrates are defined as «polyhydroxy aldehydes, polyhydroxy ketones, or substances that give such compounds on hydrolysis». But in this book we have never used chemical properties in the definition of classes. It would be better therefore to divide carbohydrates into three big groups (or sub-classes), namely, monosaccharides, oligosaccharides, and polysaccharides, and to study them separately.