The gastropods are the most diverse group of mollusks, and include snails, slugs, limpets, conchs, sea slugs, and others. They typically have a coiled body form with a twisted gut and nervous system. The coiling results in the unusual arrangement of their anus being above their head. Most have a corresponding coiled shell. Coils are added as the gastropod grows, and they tend to get larger, making a cone shape. Some shells are adorned with spines, grooves, or other textures that add strength. Many have a hard flap (operculum) on the foot that can be closed over the shell opening like a trap door. The shape of a gastropod shell often can be used to identify species. A gastropod's shell not only protects it from predation, but also keeps land species (terrestrial) from drying out. Some terrestrial gastropods that live in humid places, such as slugs, have a thin shell or no shell at all.

Mollusks have soft bodies (mollis = soft) with no internal skeleton. They hold their shape by internal water pressure (a hydrostatic skeleton). A muscular skin-like structure called the mantle covers the back of a mollusk, protecting its mass of internal body organs (viscera). Most mollusks also have a hard shell or at least some hard plates over the mantle. Shells are made of a protein matrix holding together crystals of calcium carbonate. Under those layers is a calcium-containing third layer that in some species is shiny mother-of-pearl. This layered structure makes for a strong shell that protects the soft parts from predators and provides a site for muscle attachment. Most mollusks move their bodies slowly using a muscular structure called the foot to creep along, stick to, or burrow into surfaces, although some mollusks (e.g. squid and scallops) swim.

Mollusks have been exploited by people around the world for thousands of years. Both shelled (snails, clams, scallops, conchs) and unshelled mollusks (squid and octopus) have been popular food items since prehistoric times. Several early societies used shells such as cowries for money. Today, mollusk shells are often collected and sold. Even mollusk waste products have value. A pearl is just shell layers that the mollusk uses to cover debris that gets under its shell. All shelled mollusks make them, but it is the pearl oysters that sometimes make the symmetrical, shiny ones, essentially decorative pieces of dirt. Over the years, mollusks have been used for many other purposes: dyes, decorative inlays, medicines, blades, fishing lures, tweezers, and horns. Overharvesting has endangered many mollusks species, and cultivation of mollusks had emerged as one solution.