When organizing the big family tree of everything that's alive, you use clades, which means a group that all of the individuals in it have the same common ancestor. E.g. All vertebrates, wether mammals, reptiles, etc, have the same vertebrate ancestor. Mammals also share the same tetrapod ancestor, so they're all tetrapods.

Fish doesn't work like that, because we don't count all the its ancestors as fish (tetrapods have a common bony fish ancestor, for example, but you wouldn't call a parrot a fish). But you know what a fish is. We call this a paraphyletic group.