Sodium benzoate is a salt of the aromatic acid - benzoic acid. It is a widely used preservative in the food and cosmetic industry. In the food industry it is usually known as E211. Although benzoic acid is a more effective preservative, it is not very soluble in cold water and therefore the more soluble sodium benzoate is often used instead.
Sodium benzoate is found naturally in cranberries, apples and cinnamon. The sodium benzoate used in cosmetics is derived synthetically. Besides working as a preservative it is also known to be a fragrance ingredient, anti-corrosive agent and masking ingredient.
There has been some controversy in the past years about sodium benzoate being a carcinogenic product. Research showed that if it interacts with ascorbic acid (vit C) it can potentially form benzene which is carcinogenic. For this to happen though it would require excessive heat exposure. Alone sodium benzoate is not a toxic or carcinogenic ingredient.