"It's kind of like cod," is the easy answer. And while pollock and haddock (pictured below) are in the same family as cod, though they do have some differences.
Pollock can be distinguished by their greenish coloring, and have a white lateral line. They have a slightly stronger flavor, with a larger and firmer flake. Haddock, on the other hand, have a black lateral line and can be distinguished by a black "thumbprint"on the side of their body. They have a finer flake and more mild flavor.
Another common whitefish in the northeast, the white hake, is not a part of the same family, though is in the same order (Gadiformes). They have a more slender body with softer flesh. And lastly there's the Silver Hake, also known as Whiting, which is significantly smaller, averaging 1/2-1lb each. They are silvery in color with sharp teeth. Their flesh is softer and less flaky, and they're best cleaned up and fried whole.