Three notable classes of the phylum Mollusca are:

Gastropods-
This class contains organisms such as snails and slugs. Species live on land and in sea.
Thus is an example, the slug:

Cephalopods-
This class is the most intelligent of all invertebrates. It contains the octopus, squid, cuttlefish, and the chambered nautilus.
Examples are thus, the octopus and chambered nautilus:

Bivalves-
This class is straight out of an Eastern Shore seafood restaurant. It includes clams, oysters, mussels, and scallops. These are soft filtering animals that live inside two shells hinged together.
An example (photo stolen from wikipedia), clams:

All photos taken by me except for the one above