Xylitol is an organic replacement for sugar. It doesn't impact insulin levels, and is used in lots of products. However, it's harmful to pets.
NaturallySavvy.com says this:
Poison Control states that 3 grams of Xylitol is enough to kill a 65 lb dog. To put this into perspective, little mints contain an average of .05 grams per mint. This means that your 65 lb dog would have to only eat 6 small mints to die. If he were eating gum at .72 grams per piece, 3-4 pieces could cause severe symptoms and probably death.
Please make sure you keep all types of food, snacks, even gums and mints, somewhere that your pet can't get to them. Don't leave all that extra Halloween candy laying around. There are many things that humans use to their benefit that can be very dangerous and even deadly to an animal.
Also, check out Snopes.com for more information.