The cuttlefish is a cephalopod related to the squid. The flesh of the cuttlefish is meatier and has a more complex flavour. Both the cuttlefish and the squid have an ink sac. The liquid it contains can be used in the cooking process but it should be added towards the end or it will coagulate. The ink of the cuttlefish is generally felt to be richer, denser, smoother than that of the squid. Generally cuttlefish are held to be superior, but the Brits are rather nervous about this and call just about all of them are referred to as squid in error, and they are pretty squeamish about them in the first place!