Gastropods vary considerably in structure and way of life. The smallest species are barely visible, whereas the California black sea hare weighs as much as 14 kg (30 lb). Evolutionarily the animals are successful, being common in most marine and freshwater habitats and the only mollusks to flourish on land. The three gastropod subclasses are the Prosobranchia, the Opisthobranchia, and the Pulmonata, described below.

The ancestors of gastropods had bilateral symmetry; that is, they had right and left sides. The animals evolved, however, so as to become asymmetric. This happened through two processes, the first of which was torsion, a twisting of the body. Originally the gills and anus of a gastropod were at the hind end of the body. The left half of the body began to increase in relative size, however, and the upper part of the body (including the shell) rotated like a turret so that the gills and anus were now above the head. The change may have taken place to protect the head or make it easier for the body to balance the shell.

The second process leading to asymmetry was the development of a coiled shell, which usually spirals to one side. Not all shells followed this pattern; sometimes the shell is cap-shaped, with little or no coiling involved. The latter kind of snail, called a limpet, can cling to rocks or shells with its broad foot. When the shell is deep, however, it is usually coiled, and the snail can then crawl about freely. If it needs protection, it can withdraw into the shell and close the opening with a doorlike structure, called an operculum, on the foot. A further development is seen in slugs, which have lost or reduced the shell. Young slugs usually have well-developed shells, but these are either shed or kept as a small remnant in the adult. Both snails and slugs crawl slowly, mainly using waves of muscular contraction of the single foot; some, however, can swim.

The gut of a gastropod is a coiled tube with various glands and sometimes a gizzard. The nervous, circulatory, and reproductive systems are well developed.

Lower gastropods have separate sexes and reproduce by spawning eggs into the water, where they are fertilized with sperm and develop. The young larvae swim about, settle, and mature. In advanced gastropods fertilization takes place internally, and coverings are produced that protect the eggs and young, which are sometimes also guarded by the female. At times the whole development process is internal.

In the more modified subclasses Opisthobranchia and Pulmonata, the animals are almost always hermaphroditic (containing both male and female reproductive organs). This allows them to mate with any mature animal of the same species. In some Pulmonata an ability to self-fertilize is common, and a few species of snails can reproduce without fertilization of the eggs.

The abundant and diverse gastropods are an important part of the food web, whether as herbivores, carnivores, or omnivores. Some are parasites, and many specialize by feeding on unusual, hard to eat, or poorly digestible materials. They are also important as a source of food for other animals. Among the gastropods eaten by humans are the abalone, other marine snails, such as the conch and the periwinkle, and land snails of various types. The abalone is taken commercially and as sport.

A few gastropods are harmful. Some snails and slugs damage crops and garden plants, and others are pests in oyster beds. In some parts of the world, freshwater snails harbor blood flukes, worms that cause a serious disease in humans.

The first gastropods appeared in the early Cambrian period, about 600 million years ago. The most primitive living gastropods are of the subclass Prosobranchia and are mostly marine, with a few freshwater and terrestrial species. The three suborders are Archaeogastropoda (archaic forms such as abalone and limpets), Mesogastropoda, and Neogastropoda (advanced forms such as oyster drills and cone shells).

The subclass Opisthobranchia is almost wholly marine. The shell tends to be reduced, and the gill migrates toward the rear of the body. Eight orders exist, including the less modified tectibranches (bubble shells, sea hares, and allies), the shell-less nudibranchs, and two groups of pteropods that swim in the plankton.

In the subclass Pulmonata the mantle cavity has become a lung, and the operculum is lost. The group has a few marine forms. Most terrestrial snails and slugs belong to the order Stylommatophora, and most freshwater snails belong to Basommatophora.