Not that it's the best source, but it's easy - Wikipedia:
Corn syrup is a syrup, made using cornstarch as a feedstock, and composed mainly of glucose. A series of two enzymatic reactions are used to convert the corn starch to corn syrup. Its major uses in commercially-prepared foods are as a thickener, sweetener, and for its moisture-retaining (humectant) properties which keep foods moist and help to maintain freshness.
HFCS is a variant in which other enzymes are used to convert some of the glucose into fructose. The resulting syrup is sweeter and more soluble.
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