Snail is a common name loosely applied to shelled gastropods. The name is most often applied to land snails, terrestrial pulmonate
gastropod molluscs. However, the common name snail is also used for most of the members of the molluscan class Gastropoda that have a coiled shell that is large enough for the animal to retract completely into. When the word "snail" is used in this most general sense, it includes not just land snails but also numerous species of sea snails and
freshwater snails. Gastropods that naturally lack a shell , or have only an internal shell, are mostly called slugs, and land snails that have only a very small shell (that they cannot retract into) are often called semi-slugs. Snails have considerable human relevance, including as food items, as pests, as vectors of disease, and their shells are used as decorative objects and are incorporated into jewelry. The snail has also had some cultural significance, and has been used as a metaphor. Snails that respire using a lung belong to the group Pulmonata. As traditionally defined, the Pulmonata were found to be
polyphyletic in a molecular study per Jörger
et al., dating from 2010.  . But snails with
gills also form a polyphyletic group; in other words, snails with lungs and snails with gills form a number of taxonomic groups that are not necessarily more closely related to each other than they are related to some other groups.
Both snails that have lungs and snails that have gills have diversified so widely over geological time that a few species with gills can be found on land and numerous species with lungs can be found in freshwater. Even a few marine species have lungs.
Snails can be found in a very wide range of environments, including ditches, deserts, and the abyssal depths of the sea. Although
land snails may be more familiar to laymen,
marine snails constitute the majority of snail species, and have much greater diversity and a greater biomass. Numerous kinds of snail can also be found in fresh water.
Most snails have thousands of microscopic tooth-like structures located on a banded ribbon-like tongue called a radula. The radula works like a file, ripping food into small pieces. Many snails are herbivorous , eating plants or rasping algae from surfaces with their radulae, though a few land species and many marine species are
omnivores or predatory carnivores.
Several species of the genus Achatina and related genera are known as giant African land snails; some grow to 15 in (38 cm) from snout to tail, and weigh 1 kg (2 lb).  The largest living species of sea snail is
Syrinx aruanus ; its shell can measure up to 90 cm (35 in) in length, and the whole animal with the shell can weigh up to 18 kg (40 lb).