The class Gastropoda or gastropods (also previously known as univalves) are members of the phylum Mollusca and are more commonly known as "snails and slugs". Gastropoda include thousands of species of marine snails and sea slugs, as well as freshwater snails and freshwater limpets, and the terrestrial (land) snails and slugs. This class of animals is second only to insects in its number of known species.

Although the name "snail" can be, and often is, applied to all the members of this class, very commonly this word is restricted to those species which have an external shell large enough that the soft parts can withdraw completely into it. Those gastropods without a shell, and those which have only a very reduced or internal shell, are often known as slugs. The marine shelled species of gastropod include edible species such as abalone, conches, periwinkles, whelks, and numerous other sea snails which have coiled seashells. There are also a number of families of species such as all the various limpets, where the shell is coiled only in the larval stage, and is a simple conical structure after that.