In non-scientific use, the term sugar refers to sucrose (also called "table sugar" or "saccharose") — a white crystalline solid disaccharide. Humans most commonly use sucrose as their sugar of choice for altering the flavor and properties (such as mouthfeel, preservation, and texture) of beverages and food. Commercially-produced table sugar comes either from sugar-cane or from sugar-beet. Manufacturing and preparing food may involve other sugars, including palm sugar and fructose, generally obtained from fruit.
In this informal sense, the word "sugar" principally refers to crystalline sugars; but a great many foods exist which principally contain sugar: these generally appear as syrups, or have specific names such as "honey" or "molasses". Many of these comprise mostly sugar; and sugar may dissolve in water to form a syrup.
Scientifically, sugar refers to any monosaccharide or disaccharide. Monosaccharides (also called "simple sugars"), such as glucose, store chemical energy which biological cells convert to other types of energy.
Under an optical microscope at low power, sugar will look like a bunch of messy shapes scattered about. It will feature fragments and pieces sticking out from one another, differing in size and in shape.
In a list of ingredients, any word that ends with "ose" will likely denote a sugar. Sometimes such words may also refer to any types of carbohydrates soluble in water.
In culinary terms, the foodstuff known as sugar delivers a primary taste sensation of sweetness. Apart from the many forms of sugar and of sugar-containing foodstuffs, alternative non-sugar-based sweeteners exist, and particularly interest people who have problems with their blood sugar level (such as diabetics) and people who wish to limit their calorie-intake, but while enjoying sweet foods to a greater degree. Both natural and synthetic examples exist with no significant carbohydrate (calorie) content, for instance stevia (a herb) and saccharin (produced from naturally occurring but not necessarily naturally edible substances by inducing appropriate chemical reactions).