What is Monosaccharides?

A monosaccharide is a polyhydroxy aldehyde or ketone, usually $\ce{C_nH_{2n}O_n}$ with $n$ from 3 to 7, that cannot be hydrolysed into smaller sugars. An aldose has an aldehyde group at C1, a ketose a ketone group, usually at C2. Glucose is an aldohexose, fructose a ketohexose.