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.

Most mollusks have a "radula", a ribbon made of chitin with rows of teeth (denticles). The radula is always used to feed, but how it is used varies widely. Radulas are specialized to the diets of mollusks, which range from fully carnivorous to entirely herbivorous. The radula may be used to filter, scrape, crush, cut, or stab, depending what food is eaten. Predatory murexes use the radula to drill holes into other mollusks, whereas limpets use it to scrape algae off rocks. The shape of the radula and denticles can be used to figure out what mollusk it came from. Nudibranchs that feed on corals have long, skinny denticles for scraping the thin layer of flesh off the coral skeleton. Queen conchs have a comb-like radula with thousands of tiny denticles for filtering small food from the water. Regardless, as denticles wear away, they are continuously replaced from top to bottom.