Splenda (sucralose) is made from sucrose, but chemically altered by adding chlorine ( making it a chlorinated disaccharide, if you will). This prevents the body from actually metabolizing it like a sugar; in fact, it is eliminated from the body completely unmetabolzied.
some more info, via cut n' paste:
Sucralose: Also known by its trade name, Splenda, sucralose is 600 times sweeter than sugar. After reviewing more than 110 animal and human safety studies conducted over 20 years, FDA approved it in 1998 as a tabletop sweetener and for use in products such as baked goods, nonalcoholic beverages, chewing gum, frozen dairy desserts, fruit juices, and gelatins. Earlier this year, FDA amended its regulation to allow sucralose as a general-purpose sweetener for all foods.
Sucralose tastes like sugar because it is made from table sugar. But it cannot be digested, so it adds no calories to food. Because sucralose is so much sweeter than sugar, it is bulked up with maltodextrin, a starchy powder, so it will measure more like sugar. It has good shelf life and doesn't degrade when exposed to heat. Numerous studies have shown that sucralose does not affect blood glucose levels, making it an option for diabetics.