These are processed fish cakes made from pureed fish pieces shaped into cakes. The best quality fish cakes have a high fish content. Some brands contain MSG, so always check the ingredients on the back of the package to see what you’re getting. To be totally sure, you can make fish cakes at home using only the ingredients you want.
Some people call these “fish loaves” or “fish sausages,” and in Japan they call them kamaboko. Koreans sometimes call them odeng (오뎅) which is a loan word from the Japanese oden (おでん). In China they call the round ones “fish balls.”