The fish used is mostly Pollack.
It's a type of fish cake, basically. It's white fish meat ground into a paste and formed. Surimi is made into things that aren't imitation crab, too! Fish balls and fish cakes and all sorts of things.
What is enough to know about this, that it is blended paste, which means it gradually becomes composed of the cheapest available ingredients as the cost is continually lowered, while the experience kept up using extenders, like bologna for example. This is about this kind of a food you have to know. Happy distressing!