so many folks didn't pay attention in biology class....LOL
Stingrays are fish because there are three different groups of fish. Pet fish like goldfish, what we commonly think of as a fish, are only a part of one group, bony fish. Also, there are Jawless fish, which have long, slender bodies and no jaws, something like an eel. The last group, cartilaginous fish, house the sharks and rays because they have cartilage, not bone, for skeletons.
The stingray is a fish because it has the characteristics of a fish. These are that it is aquatic and a vertebrate, that it is ectothermic (cold-blooded), and their limbs are modified into fins for swimming. Generally, they will have scales, but cartilaginous fish have far fewer, less noticable scales on their body that do not completely resemble what a goldfish's scales look like.
By fins, the stingray has rays off to the sides of its body which are considered fins. Some people will call them wings.