Stevia is a sugar substitute made from the leaves of the Stevia rebaudiana plant of South America. The leaves contain chemicals called steviol glycosides that have a highly concentrated sweet flavor. 

Stevia is  about 200 to 400 times sweeter than table sugar and is a non-nutritive sweetener, which means it has no carbohydrates, calories, or artificial ingredients.

Stevia: Health Benefits and Risks (webmd.com)