Squids are actually classified as being both mollusks and cephalopods. There are literally tens of thousands of different species of mollusks with squids being just one of them. Part of what makes mollusks so unique is the fact that they have a variety of forms, more so than any other phylum of animal.
Some of the different animals in this classification include snails, clams, slugs, and a number of other gastropods. Typically mollusks are very diverse and located in temperate as well as tropical regions around the globe. One interesting fact is that the giant squid is actually one of the very largest invertebrates there are.