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 usually known as limpets.
The class 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 that 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.