Stevia is a zero-calorie sweetener made from rebaudioside A, one of the components of the Stevia rebaudiana plant native to the tropical parts of North and South America. It's sold under brand names like Truvia and Purevia, or as a dietary supplement in the form of a liquid extract. It's used in very small amounts because it is approximately 250 times sweeter than table sugar, so it's good for low-calorie diets or to maintain weight loss.
Stevia is not poisonous and it does not appear to cause cancer. There may be some confusion with research performed on rats using steviosides, which led to tumor growth. Stevia rebaudiana contains both rebaudioside A and steviosides, but no studies have shown any danger when consuming rebaudioside A. Stevia has been used safely as a sweetener in Japan for several decades and many people in the United States have used the dietary supplement form of stevia without any apparent problems.
Powdered stevia is sold in jars and in individual packets and can be used just like table sugar. Stevia is stable at high temperatures so it can also be used in cooking and baking. Some products include additional ingredients, such as erythritol, in order to add bulk, which makes the sweetener easier to measure. You can also purchase stevia extract in liquid form, but it can be tricky to use because you need only a small amount. Using too much results in a bitter flavor. You can also make your own liquid stevia sweetener with powdered stevia leaves.
Other No-Calorie and Reduced Calorie Sweeteners
Academy of Nutrition and Dietetics. "Position of the Academy of Nutrition and Dietetics: use of nutritive and nonnutritive sweeteners." J Am Diet Assoc. 2004 Feb;104(2):255-75.