Corn syrup is a syrup made from corn. It is naturally composed mainly of glucose, but usually modified by enzymes to contain between 42% and 90% fructose. When used as a sweetener, corn syrup has a taste that makes it inferior to sugar. Corn syrup can also affect some people who are highly allergic to corn. Corn syrup can be less expensive than sugar though, primarily due to taxes and subsidies. Many commercial products made in the USA are reformulated to use corn syrup as production increases.
Corn syrup can be useful in home cooking as well. It is less likely to crystalize than plain sugar, leading to usage in frosting and fudge. It is a liquid and thus does not need to be dissolved.