Answer to this question needs to be very long but here is a short story.
Monosaccharide is a simple sugar consist only of one unit. They serve as building blocks for more complex carbohydrate forms.
For example:
Glucose
Fructose
Galactose
Ribose and Deoxyribose – RNA DNA
and so on…
Disaccharides are a group of sugars composed of two monosaccharide groups linked together.
For example:
Maltose = Glucose + Glucose
Sucrose = Glucose + Fructose
Lactose = Glucose + Galactose
Polysaccharides are complex composed of numerous monosaccharides combined through the loss of water molecules.
For example:
Starch
Amylopectin
Glycogen
…
