Monosaccharides are carbohydrates which cannot be hydrolyzed to simple molecules. They may contain 3-7 carbon atoms but monosaccharides containing five and six carbon atoms are more abundant in nature. Those containing an aldehyde group are called aldoses while those containing a keto group are called ketoses. The aldehyde group is always present at C₁ while the keto group is usually present at C₂.  All monosaccharides reduce Tollens reagent as well as Fehling's solution and hence are called reducing sugars. Pentoses ands hexoses have cyclic structures, furanose (five membered) and pyranose (six membered). Ribose in RNA and fructose in sucrose exist in the furanose form while glucose, mannose, galactose, etc. all exist in the pyranose form. During ring formation C₁ in aldohexoses and C₂ in fructose becomes chiral and hence all these monosaccharides exist in two stereoisomeric forms called the  α-anomer and the β-anomer while C₁ and C₂ are called glycosidic or anomeric carbon atoms and the bonds connected to glycosidic carbon are called glycosidic linkages. In contrast, stereoisomers, which differ in configuration at any other chiral carbon are called epimers. When two molecule of the same or different monosaccharide combine together through glycosidic linkage, a disaccharide is formed. All monosaccharides and reducing disaccharides react with excess of phenyl hydrazine to form osazones which are oftenly used for their characterization. 

In polysaccharides, the linkage connecting monosaccharide units are called 

Glycoside linkages 

In polysaccharides, the linkage connecting monosaccharides is called glycoside (glycosidic linkage). Glycosidic linkage is the covalent bond between two or more monosaccharides to form a polysaccharide. It is formed between the hemiacetal or hemiketal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some compound such as alcohol. A substance containing a glycosidic bond is a glycoside.