A domesticated version of the wild olive-green type, goldfish have multiple genetic variations in color, body shape, size, finnage, and eye shape.
They can have the colors black, brown, white, red, orange, blue, and metallic versions of those colors. A goldfish with a single tail fin (cadual fin) and normal fish-shaped body is either a comet or a shubunkin. All the rest have double tail fins and shortened, squat bodies. Some of them have head growths, bulging eyes, no dorsal fin, nasal growths, and other weird features. Koi are another domesticated fish similar to goldfish. Goldfish are cool-water dwellers, and prefer neutral pH water of temperatures in the 60's. These are often the first (and more often, the last) fish that people own when venturing into the hobby of fishkeeping. Unfortunately, because you actually need to read a book to know how to take care of fish, most people aren't successful at it and drop the hobby.