Octopus is good food, grilled, sauteed, in a cold salad, whatever. We probably learned in high school bio that they are molluscs, same as squid. Cephalopod molluscs.
Compared to shelled molluscs, these creatures are advanced and have multiple brains. As far as eating goes, with squid we gut them and eat the rest, but with polpo we cook just the feet. I have never heard of eating an octopus head and guts as we happily do with clams and oysters (which have no heads, really. No eyes - but scallops have sort-of eyes...).
You can read about octopus anatomy here.