Sugar alcohols are types of sugars found in fruits and vegitables, but commercially they are made from glucose and have a slightly different chemical structure than sugar. That makes them taste almost as sweet as sugar.
Sugar alcohols are absorbed slowly and incompletely, so they're considered reduced-calorie or no-calorie sweeteners.
That doesn't mean products made with sugar alcohols are calorie free, because they may contain other carbohydrates or added fat.
Some products, in fact, such as sugar free cookies and candies have just as many calories as their sugar-laden counterparts.
This was an article in the Atlanta Journal a few weeks ago. The quotes were from Chris Rosenbloom of the College of Health and Human Sciences of Georgia State University.