It's syrup made from corn starch, made up of mostly glucose, that has been through an "enzymatic" process to turn the glucose into fructose. The fructose is then mixed with 100% glucose until it reaches desired sweetness. It's commonly used in processed foods to replace sugar.
Corn Syrups have become more popular because they are cheaper to transport and easier to mix with products b/c it's a liquid.
As far as health concerns? The results are inconclusive, but some studies suggest that high-fructose corn syrup contributes to obesity as well as other diseases:
"In a 2007 study, rats were fed a diet high in fat and HFCS and kept them relatively sedentary for 16 weeks in an attempt to emulate the diet and lifestyle of many Americans. The rats were not forced to eat, but were able to eat as much as they wanted; they consumed a large amount of food, suggesting that fructose suppresses the sensation of fullness. Within four weeks, the rats showed early signs of fatty liver disease and type II diabetes."
- http://en.wikipedia.org/wiki/High-fructose_corn_syrup