Dr Cohen: An artificial sweetener is anything that has less nutritional value than ordinary sugars such as saccharose (the most well-known), glucose, fructose and lactose.

There are two types of sweetener: 
-nutritional sweeteners, which give you the same sugar hit as 'ordinary' sugars in the same quantity, but contain 2.4 calories per gram. They're also known as polyols, and examples are sorbitol, mannitol, xylitol and maltitol. They're often used in sweets.
-The other type are known as intense sweeteners because they're very strong but come in very small quantities, and hardly contain any calories. Examples are  aspartame, acesulfame potassium, saccharine and cyclamate.