The word snail is a common name that can be used for almost all members of the molluscan class Gastropoda which have coiled shells in the adult stage. (Those snails which do not have a shell or only a very small shell are usually called slugs. Snails which have a broadly conical shell which is not coiled, or appears not to be coiled, are most often known as limpets.)

The class of Gastropoda (the snails and slugs) is second only to the insects in terms of total number of species. Snails are extraordinarily diverse in habitat, form, behavior, and anatomy, and therefore what is true of one snail species may not at all be true of another.

Snails can be found in a wide range of different environments from ditches to deserts to the abyssal depths of the sea. The great majority of snail species are marine. Many others are terrestrial, and numerous kinds can be found in fresh water, and even brackish water. Many snails are herbivorous, though a few land species and many marine species are omnivores or predatory carnivores.

Although the average person might perhaps be more familiar with terrestrial snails, land snails are in the minority. Marine snails have much greater diversity, and a greater biomass. Snails which respire using a lung belong to the group Pulmonata, while those with gills form a paraphyletic group, in other words, snails with gills are divided into a number of taxonomic groups that are not very closely related.

Snails with lungs and with gills have diversified widely enough over geological time that a few species with gills can be found on land, numerous species with a lung can be found in freshwater, and a few species with a lung can be found in the sea.

Although the word snail is often used for all shelled gastropods, the word "snail" can also be used in a much more limited sense, to mean various larger species of air-breathing (pulmonate) land snails. The majority of this article is about air-breathing land snails.

Land snails

Species of land snails live in almost every kind of habitat, from deserts and mountains to marshes, woodland, and gardens. However, certain species are "anthropophilic", which means they are found most often around human habitation.

Whichever reasonably large land snail species is most commonly seen or most commonly eaten in a given area, that species will usually be referred to simply as "snails" by the local people. In many parts of the world, the fairly large edible species Helix aspersa or Cornu aspersum, has been introduced, and has become a pest in farms and gardens, so this is perhaps a good example of a species commonly known as "the snail."

Slugs

main: Slug

Gastropod species which lack a conspicuous shell are commonly called slugs rather than snails, although, other than having a reduced shell or no shell at all, there are really no appreciable differences between a slug and a snail except in habitat and behavior. A shell-less animal is much more maneuverable, and thus even quite large land slugs can take advantage of habitats or retreats with very little space  places that would be totally inaccessible to a similar-sized snail, such as under loose bark on trees or under stone slabs, logs or wooden boards lying on the ground.