Xylitol is an artificial sweetener that is relatively well tolerated by most people. It is in lots of "sugar-free" or artificially sweetened products, from candy to gum to nutritional supplements like melatonin.
Now there are a few brands of peanut butter that contain it. Xylitol in pets can cause life-threatening episodes of low blood sugar, and can cause severe, even fatal liver damage.
Here's a link to more about the Xylitol in some peanut butter: