Stevia is the common name for extracts from the plant Stevia rebaudiana. In the U.S., a purified component form of the plant — called rebaudioside A (rebiana) — is "generally recognized as safe" by the Food and Drug Administration (FDA) and may be used as an artificial sweetener in foods and beverages.
Refined stevia preparations (Pure Via, Truvia, others) are considered nonnutritive sweeteners — they have virtually no calories — and so may appeal to people trying to lose weight. But there's no evidence that they offer an advantage for weight loss over other nonnutritive sweeteners. In addition, these highly refined stevia extracts may cause mild side effects, such as nausea or a feeling of fullness.
The FDA hasn't approved whole-leaf stevia or crude stevia extracts for use as food additives because of concerns about possible health effects. In particular, the FDA has concerns about the effects of whole-leaf or crude stevia on blood sugar control, the kidneys, and the cardiovascular and reproductive systems.
Remember that while sugar substitutes, such as refined stevia preparations, may help with weight management, they aren't a magic bullet and should be used only in moderation. If you eat too many sugar-free foods, you can still gain weight if these foods have other ingredients that contain calories.