High-fructose corn syrup (HFCS) is made from corn starch whereas sucrose us usually made from sugarcane or beets. Because starch itself is a simple sugar, a chain of glucose molecules joined together, once it's broken down into individual glucose molecules, it's corn syrup, which is essentially 100% glucose. It becomes high-fructose corn syrup when enzymes are added.
HFCS is sometimes called isoglucose and is used in the food industry to sweeten processed foods such as soft drinks, creams, cakes, confectionery, yogurts etc. to keep costs lower. It's sweeter than regular sugar because the fructose level can be more like honey.