Snails are invertebrates, which means they lack a backbone. They belong to a large and highly diverse group of invertebrates known as the Phylum Mollusca (also known more commonly as 'mollusks'). The Phylum Mollusca includes slugs, clams, oysters, mussels, squids, octopuses, and nautiluses, in addition to snails.
In scientific terms, snails are classified into the following hierarchy of animal groups:
Kingdom: Animalia (Animals)
Phylum: Mollusca (Mollusks)
Class: Gastropoda (Gastropods)
Order: Stylommatophora (Terrestrial Snails and Slugs)
What Classification Reveals About Terrestrial Snails
Each of these levels of classification tells us a bit about snails in increasingly specific terms. For example, because snails are animals, they possess the characteristics shared by most animals: they are multicellular and are heterotrophs.
Since terrestrial snails are classified as mollusks and gastropods, they posses the typical characteristics of those groups too. They have a single, often spirally coiled shell (univalve), they undergo a developmental process called torsion, and they possess a mantle and a muscular foot used for locomotion.
Finally, because terrestrial snails are grouped with terrestrial slugs into the Order Stylommatophora, they share some similarities with terrestrial slugs. Because sea slugs and sea hares are in a different order, terrestrial snails have less in common with sea slugs and sea hares than they do with terrestrial slugs. Members of the Order Stylommatophora are characterized by having eyes on the top of tentacles (sea snails have eyes at the base of their tentacles).
Terrestrial snails belong to a group within the Order Stylommatophora called the 'pulmonates'. This classification level is as yet unassigned (it is not officially a family) but the term pulmonate is used to refer to air-breathing land snails.