thumb|250px|Helix pomatia The word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails. Snails lacking a shell or having only a very small one are usually called slugs. Snails that have a broadly conical shell that is not coiled or appears not to be coiled are often known as limpets.

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

Snails can be found in a wide range of environments from ditches, deserts, and the abyssal depths of the sea. Although most people are familiar with terrestrial snails, land snails are in the minority. Marine snails have much greater diversity and a greater biomass. The great majority of snail species are marine. 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.

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.

Land snails

Although the word snail is often used for all shelled gastropods, the word "snail" can also be used in a more limited sense to mean any of several species of large, air-breathing (pulmonate) land snails. Whichever land snail species is most commonly seen or most commonly eaten in a given area will usually be referred to as "snails" by the local people.

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.

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 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.