Carbohydrates are composed of Carbon, Hydrogen and Oxygen. 

Hydrogen and Oxygen are composed in the same ratio as water(2:1).

Carbohydrates are broadly classified into two groups,

1. Monosaccharides(Simple carbohydrates or simple sugars) with small molecules.

Major monosaccharides:

Glucose is absorbed directly by the body, without any breakdown. Galactose and fructose are converted into glucose by liver, before their utilization by body cells. 

Surplus glucose is converted and stored in the form of glycogen and fat by liver, muscles, and fat cells.

Hydrolysis breaks glycogen to form glucose monomers.

2. Polysaccharides(Complex carbohydrates or Starches) are polymers formed by combination of monosaccharides through covalent bonds. Polysaccharides are insoluble in water.

Polysaccharides either serve as storage material(glycogen) or forms building material(cellulose, chitin etc..) for organisms' structures.

Oligosaccharides: carbohydrates containing between two and ten monosaccharide units linked together.

Disaccharides(double sugars) -  formed by combination of two monosaccharides joined by a covalent bond.

Major disaccharides:

These complex sugars are broken down into simple sugars before their absorption.