What is the difference between a fish and a squid?

The only biological answer is that both have a small internal support. in squids this is thin uncalcified material and is known as a pen. in cuttlefish it it a hard bony plate. people will also say things like that cuttlefish are broader, or have eyes towards the front of their head but there are so many variations between species of cephalopods that this is not reliable. hope this helps.