The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, and the structure of the nervous system. Other than these things, molluscs express great morphological diversity.
A striking feature of molluscs is the use of the same organ for multiple functions. For example, the heart and kidneys are important parts of the reproductive system, as well as the circulatory and excretory systems; in bivalves, the gills both "breathe" and produce a water current in the mantle cavity, which is important for excretion and reproduction. In reproduction, molluscs may change gender to accommodate the other breeding partner.
Molluscs have been and still are an important food source for humans, but with a risk of food poisoning from toxins that accumulate in molluscs under certain conditions. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as money in some preindustrial societies.
Today, most molluscs are aquaculture products, not wild.