What the frick is a fish!?

So its not a reptile, nor is it a mammal or amphibian! What the frick are they!? And not whales or dolphins, the real frickin fish that have real names like trout and bass and even cat fish but they are commonly introduced as 'holy crap, you actually caught a fish!' Also what is a cephalopod? Its what an octopus is but what does it mean?