They are called carbohydrates because they are considered as hydrates of carbon. Carbohydrates are classified as monosaccharides, oligosaccharides and polysaccharides. Monosaccharides and oligosaccharides together are called sugars. Polysaccharides are known as non-sugars. Sugars are crystalline carbohydrates, soluable in water having a sweet taste.
Non sugars are insoluable in water and are tasteless.
Monosaccharides are the simplest carbohydrates containing 3 to 6 carbon atoms which cannot further be hydrolyzed into simpler units. Oligosaccharides are carbohydrates obtained from the condensation of 2 to 9 monosaccharide molecules and upon hydrolysis yield the monosaccharide from which they were formed. Dissacharides are formed by the condensation of two monosaccharide molecules by the elimination of a molecule of water. These can be hydrolyzed into two monosaccharide molecules. Polysaccharides are carbohydrates formed by the condensation of a very large number of monosaccharide molecules, usually hexose molecules.