Squid and cuttlefish belong to the cephalopod family, a sub-species of mollusc that also includes octopus (which is difficult to find in Hong Kong).
What are they? Squid and cuttlefish are edible molluscs without an external shell (squid have a transparent quill in their bodies; cuttlefish have a harder 'bone'). When frightened, cuttlefish and squid (and octopus) expel ink, which is edible and used to colour and flavour pasta, rice and other dishes.
What are the differences? Cuttlefish have proportionately larger, thicker and rounder bodies than squid, which are more elongated. All but the smallest specimens need to be cleaned (innards and skin removed). Smaller squid and cuttlefish are more tender than larger specimens. You'll usually see live cuttlefish (they're cute creatures) in fishtanks outside seafood restaurants; fresh (but not live) squid is sold mostly at wet markets.
What to look for? If not live, squid and cuttlefish should have a fresh, clean smell and shouldn't feel slimy or sticky.
How are they available? Fresh, frozen, salted and dried. Dried cuttlefish and squid aren't popular outside Asia, probably because they have a strong odour and flavour some people dislike.
What else? Cuttlefish bones are given to birds as a source of calcium.