The simplest carbohydrates are sugars or saccharides, they are polyhydroxylated aldehydes or ketones. They are classified as aldoses, if they have the aldehyde function, and ketoses, if they have the ketone function.
The aldoses are classified depending on the number of carbons in: aldotrioses, aldotetroses, aldopentoses and aldohexoses.

Aldose of three carbon atoms (aldotriose)

Aldose of four carbon atoms (aldotetrose)

Aldose of five carbon atoms (aldopentose)

Aldose of six carbon atoms (aldohexose)

Ketoses follow a similar classification.

Three-carbon ketose (ketotriose)

Ketose of four carbon atoms (ketotetrose)

Ketose with five carbon atoms (ketopentose)

Ketose with six carbon atoms (ketohexose)

Sugars are molecules that have asymmetric carbons. Glyceraldehyde (aldotriose) has one chiral center, while aldopentoses have three chiral centers...

(R)-2,3-dihydroxypropanal

(2R,3R,4R)-2,3,4,5-tetrahydroxypentanal