Knowing what the body parts of a fish are called is helpful if you need to read or talk about fish. The study of body parts of animals and plants (and knowing what each part does) is called "Anatomy". Vets have to know this stuff.  Fish are not really all that different from other animals except they have fins instead of legs and they breath through gills instead lungs. Have a look at the picture and you will see that you already know the names of some fish body parts.  The special name for each type of fin might be new - see if you can remember what each one is called.