Corn sugar, also known as priming sugar, is an inexpensive fermentable sugar that produces alcohol without leaving the cloying residual sweetness of malt extract.
Adding a small amount (no more than 10% of total fermentables) will increase alcohol content with minimal impact on taste.
Adding 30-40% will produce a beer with a considerably lighter body than one brewed strictly with malt sugars (think American light lagers).
Any canned kits, and the body of the beer they produce, can be improved upon greatly by substituting the corn sugar with malt extract, either dried or liquid.
Bottling Beer: Use 3/4 cup of corn sugar (priming sugar).
Mini Keg: Use 1/3 cup of corn sugar (priming sugar).