Stevia is a genus classification of plant in the sunflower family. Stevia rebaudiana is commonly called stevia and is found in tropical parts of North and South America. Its leaves are used in many countries as an artificial sweentener. It is a small, leafy green plant that probably could be considered a weed.
The active chemicals in stevia leaves that produce the sweet taste are called steviol glycosides. They are polysaccharides that are composed of a steviol compound and a glucose. The glycosides bind to sweet receptors on the tongue and taste sweet. In the stomach, the glucose and steviol are separated. The glucose is absorbed by bacteria in the colon for food and the steviol is not metabolized and passes through the system. This is how it is a low calorie sweetener.
Stevia can be used as glycoside extracts or whole leaves. It is not approved for use in the US by the FDA as whole leaves due to potential health concerns. The purified glycosides, however, have been approved for use and the extracts can be sold as dietary supplements.