Corn syrup is the product of cornstarch that is extracted from the pulpy center of corn kernels, mixed with natural enzymes, and broken down into glucose. These sugars are then heated to form corn syrup. Commercial corn syrup is just as sweet as granulated sugar, and can be either light or dark. Light corn syrup may have vanilla flavoring added to it, while the darker variation has its own distinctive flavor, which is similar to but lighter than molasses. Unlike sugar, corn syrup has the ability to resist crystallization, and therefore is often used in candies and some frostings. Both light and dark syrup have a natural balance of different types of sugars (dextrose, fructose, malt and glucose), which keep them stable, but corn syrup does have a limited shelf life.
Corn syrup is also not to be mistaken with high fructose corn syrup, which is a product of chemically manipulated corn syrup that is present in many soft drinks and commercial snack foods.
Because corn syrup prevents crystallization in candymaking, substituting sugar or maple syrup is not recommended. An invert sugar syrup used measure for measure, such as Lyle's Golden Syrup, makes an adequate replacement.
Store tightly sealed in a cool, dark place and it will last indefinitely; a year or more.