Sucrose is a disaccharide composed of one molecule of glucose connected via an glycosidic bond to one molecule of fructose.
Sucrose is generally extracted from sugar cane or sugar beet and then purified and crystallized. Other (minor) commercial sources are sorghum and sugar maples.
Pure sucrose is the most common sweetener in the modern, industrialized world. People, and in fact most other mammals except members of the cat family, will gladly accept a food sweetened with sucrose, even if they aren't hungry. Processed food and junk food often have sucrose added.
Sucrose has several adverse health effects. The most common is tooth decay, in which bacteria in the mouth turn sucrose into acid that attacks tooth enamel. Sucrose has a high calorie content and is also believed to cause obesity. People with diabetes mellitus need to control their intake of sucrose.
Because of the health effects of sucrose, several substitutes have been developed, although none appear to be as versatile as sugar in cooking and they may have other health consequences.