Corn sugar (mainly glucose) is a simpler type of sugar and ferments without adding flavor. Cane sugar (mainly sucrose), when used in large quantities, can create off flavors. This is because the yeast have to "invert" the cane sugar from sucrose into glucose and fructose, the process can leave off flavors in the beer. With corn sugar you have essentially already done this for the yeast and they don't have to work very hard, they can just go about their business of fermentation.
EDIT- Look for "Dextrose" or "Glucose," it is corn sugar