Sugars

The main molecule in sugar is a {carbohydrate}.
BOX on Carbohydrate structure
Carbohydrate is a compound made up of carbon, with twice as much hydrogen as oxygen called sucrose, C12H22O11. It is a disaccharide, a polymer of two simple sugars (or monosaccharides), namely, glucose C6H12O6 and fructose, of which there are several kinds, with six carbons, 10 hydrogens and 5 oxygens. Fructose is the sweetest of all naturally occurring carbohydrates, found in fruits, berries, roots and honey.
END OF BOX

Sources of sugar
In India we make sugar from sugarcane, which is cut and shredded, the juice pressed out and clarified in lime, then boiled in a partial vacuum (where its boiling point is lower) until it becomes a thick brown syrup. Water evaporates and leaves sugar crystals. The whole mass is then spun around, throwing out the syrup into molasses, leaving wet brown sugar with yeasts, fungi, bacteria, soil and plant waste. Twice again it is washed, re-dissolved, boiled, re-crystallised and spun. This is refined sugar.
Some of the boiled syrup is clarified and made into treacle.
We also make jaggery (gur in Hindi and vellam in Tamil) from sugarcane by boiling the cane juice in an open container (with a higher boiling point). A strong flavour develops and sucrose gets broken down into glucose and fructose, making it taste sweeter.
In western countries, sugar is also made from sugar beets, which have brownish-white roots. Compared to cane they have many bad-tasting and bad-smelling impurities which have to be removed. After refinement we again get plain sucrose. People making jams insist that cane sugars and beet sugars do not taste the same, but the chemical formula is the same.
We can also have sugar made from maize (you must have heard of sweet corn), but this is chemically different. The kernel of a maize corn has sugars, starches, maltose and cellulose. Starch has polysaccharides of glucose. Maltose is a disaccharide of glucose. Because fructose is not present, corn sugar is less sweet. If you ever go to the United States and drink Coca-cola there, you will feel the difference. After removing the cellulose, the corn syrup is boiled and refined to produce the sugar.
Diabetes
People whose bodies do not process enough insulin have difficulty digesting sugars, because it promotes absorption of glucose from the blood into liver and muscle cells and stored as glycogen or fats. So these people have higher blood sugar levels than normal, leading to a disease called diabetes.
These people use artificial sweeteners in their food rather than sugars,
such as saccharin, aspartame or sorbitol. These are much 
sweeter than sucrose, but a tablet contains very little of the 
sweetener, and taking large doses of the tablet is bad for health.
For example, sorbitol is used in mouthwash and toothpaste to make them 
taste sweeter.
Sugarcane is a native crop of India. Indians also discovered the process of making sugar crystals (khanda, from which we get the English word ``candy''). We know that it was available in the Gupta empire around the 5th century CE. Buddhist monks brought the knowledge of the process to China. Christopher Columbus took sugarcane to the West Indies, and many plantations came up there. These were worked by slaves from Africa. After slavery was abolished, lakhs of labourers were brought from India to the West Indies. The British government realized that instead of this, they could produce sugar in India itself, and many sugar factories came up in Bihar. Today Brazil and India are the largest producers of sugar. Brazil is the largest exporter.

Adapted from 
What Einstein told his cook
by
Robert Wolke