Corn syrup is a syrup made from maize, composed mainly of glucose. It is used to sweeten soft drinks, juices, ice cream, whole wheat bread and many other mass-produced foods. Its liquid form keeps foods moist and prevents them from quickly spoiling. In many areas it is less expensive than table sugar due to agricultural policy, e.g. the United States subsidizes its production while taxing sugar imports.
High fructose corn syrup (HFCS) is a modified form of corn syrup that has an increased level of fructose. Ordinary corn syrup is treated with the enzyme glucose isomerase which converts glucose to fructose. This process, developed by Japanese researchers in the 1970s, can increase the fructose content of corn syrup to 42%. Because fructose is much sweeter than glucose, the overall sweetness of the syrup is increased and it becomes a more useful substitute in food processing for sugar or invert sugar syrups. Fructose is also more desirable than glucose as it increases in solubility at low temperatures (so more can be concentrated per unit weight).
Through further processing, the fructose content can be increased to 55% (yielding a product that has the same sweetness as sucrose) or higher. Common commercial grades of high fructose corn syrup include fructose contents of 42%, 55%, or 90%.
Sucrose (table sugar) is a disaccharide composed of one unit each of fructose and glucose linked together. Sucrose is broken down during digestion into fructose and glucose through hydrolysis. Sucrose is 50% fructose, so HFCS may have a higher or lower fructose content than sucrose, with a corresponding change in sweetness.
Honey is another product that is a mixture of different types of sugars, water, and small amounts of other compounds. Honey typically has a fructose/glucose ratio similar to HFCS, as well as containing some sucrose and other sugars.
High fructose corn syrup is often cited by some nutritionists as one of the leading causes of obesity. (http://www.washingtonpost.com/ac2/wp-dyn/A8003-2003Mar10?language=printer) The average American consumed 62.6 pounds of high fructose corn syrup in 2001, most of which came from soft drinks. Since HFCS is used as a substitute for other sugars, particularly sucrose, in processed foods, it is not clear whether it is the chemical differences between sugars or a general increase in consumption of sugars of all types that might be linked with obesity.
Some nutritionists and natural food advocates believe that consumption of high fructose corn syrup should be avoided due to its possible links with obesity. Also cited as reasons to avoid HFCS are that it is highly refined, that it might be produced from genetically modified corn, that various molds found on corn might leave harmful byproducts in the final product, or that corn (maize) products in general should be avoided.  (http://www.bodyfueling.com/Q&A.html),  (http://www.mercola.com/2004/apr/10/corn_fat.htm) Other nutritionists say that HFCS is no more or less harmful than other forms of sugar and that all sugars should be consumed sparingly. It may be the case that confusion has arisen between the effects of consuming pure fructose as compared to pure glucose, versus the effects of consuming mixtures of the two sugars from different sources.