Sugar was "refined" in the sixth century by monks who used the purifying properties of charcoal. What we
call "table sugar" or just "sugar" is actually sucrose,
one of about 100 substances known as sugars.
Sucrose is the most common sugar found in plants, and most plants contain at least some of it. Sucrose is
a "disaccharide" with the formula
C12H22O11. There are also
such as dextrose and fructose, with the formula C6H12O6.
[there are oligosaccharides and
polysaccharides as well, but these are less familiar to most people.]
The difference between the sugars within the 2 categories is the
structure, how the atoms are arranged. Dextrose and
Fructose, for example, contain the same number and types of atoms, but the
atoms are arranged differently. This diffrence in arrangement affects how they interact with other compounds. This principle
is illustrated by the fact that while Sucrose and Lactose are both
C12H22O11, anyone not suffering from diabetes can easily digest sucrose but most adults
have trouble digesting lactose!
Sucrose is actually composed of a molecule of dextrose and fructose with a
molecule of water removed. Enzymes in human saliva cause it to absorb a molecule of water and revert back
to dextrose and fructose, in this form it is called "invert sugar".
This is essentially what honey is, the distinctive taste of different varieties being caused by various impurities.