The members of this phylum are typically called shellfish, but this phylum also includes the octopus, squids, cuttlefish, snails and slugs. Moreover, lobsters, crabs and shrimps are not molluscs, although they are sometimes also called shellfish. Generally, those with 2 external shells are called bivalves (Class Bivalves); those with a single external shell is called a gastropod (Class Gastropoda) and those without an external shell are called cephalopods (Class Cephalopoda). However, things are more complicated than that. E.g. gastropods which lack an external shell, like the slugs, as well as chitons which have a single shell, but are not gastropods.
Bivalves are so called because they have two shells enclosing the soft body. Typical members of this class are oysters, mussels, clams, cockles, etc.
Rock Oyster (Family Ostreidae) at Chek Jawa ©Tan KH. Left: Stuck to rock. Right: Together with Green Mussels
Gastropods has a single shell enclosing the soft body. Typical members of this class are snails, slugs, nudibranchs (sea slugs), etc.
Sungei Buloh. Left: Common Nerite (Nerita articulata) ©Tan KH. Right: Common Nerite with Black Chut-chut (Cerithidea quadrata) ©Lau SY