Carbohydrates, along with fats and protein, provide 100% energy to all cells in the body and are the fastest when it comes to supplying energy. [1,4] It is usually in the form of either starch or sugars in plant based foods.
The amount of energy in 1 gram differs though 4 calories in 1 g of carbohydrates and proteins, and 9 calories in a gram of fats.
It is classified into oligosaccharides, polysaccharides (starch form), monosaccharides, disaccharides (sugar form) and sugar alcohols based on the number of sugar units. 
The different naturally occurring sugar forms include glucose, galactose, fructose, sucrose, lactose, maltose, and trehalose .
Added sugars and syrups are different from naturally occurring sugars in foods and include white sugar, brown sugar, raw sugar, corn syrup, maple syrup etc. It is usually found in processed foods like soft drinks, cakes, cookies, pies, fruit, fruit punch, dairy desserts, and candy.
However, most carbohydrates occur as starches in food and are found abundantly in certain vegetables, grains, legumes etc.
Two metrics have been developed in order to rank dietary carbohydrates food sources based on their impact on the blood sugar levels in the body:- Glycemic Index (GI) and Glycemic Load (GL). [1,3] Usually, processed foods tend to have a high GI and GL.
Also, the brain is the only true carbohydrate-dependent organ and the minimum carbohydrate requirements are determined by the brain’s requirement for glucose.