Light corn syrup, according to the company that makes the market-leading Karo brand, is a mixture of corn syrup and high-fructose corn syrup (which provides increased sweetness), flavored with salt and vanilla. It is clear and colorless, with a moderately sweet flavor. In the case of corn syrup, the light is not to be confused with lite or any other misspelling meaning reduced calorie. The light refers to the shade, and, indeed, some people refer to it as white corn syrup.
Dark corn syrup is corn syrup with a small amount of refiners' syrup (a type of molasses derived from sugar cane), caramel flavor, sodium benzoate (a preservative), salt, and caramel color. Dark corn syrup has a medium brown color and a much more assertive flavor.
Both light and dark corn syrup function similarly in recipes and can usually be used interchangeably. The use of dark corn syrup will add more flavor to a recipe.