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. 

Two forms of D-glucopyranose are called 

Anomers 

Anomers are diastereoisomers of cyclic forms of sugars or similar molecules differing in the configuration at the anomeric carbon ($C1$ atom of an aldose or the $C2$ atom of a 2-ketose). The cyclic forms of carbohydrates can exist in two forms, $\alpha -$ and $\beta -$ based on the position of the substituent at the anomeric centre.