What is Sugar?Chemical and culinary definitions

What Is sugar? A chemical definition from the Exploratorium 

Britannica's take on the substance 

"All the different types of sugar"A culinary definition from Master Class

“Rose’s Sugar Bible" by pastry chef Rose Levy Berenbaum, author of The Cake Bible, explains the multiple varieties of this baking essential.

What's the difference between sugar in fruit and sugar in sweets? Britannica explains it, in a nutshell.