Oysters are a type of bivalve (clam) that are themselves a type of mollusc (Phylum Mollusca, Class Bivalvia). They have been on Earth since about the Jurassic Period. Although they technically have the bilateral symmetry typical of bivalves, most oysters have more irregular shapes because they commonly encrust on surfaces including other oysters. Oysters reproduce sexually, but can switch sexes throughout their lives. The egg and sperm are expelled from the oyster, fuse into a zygote in the water and have a free-swimming larval stage for a while. When they first settle on a surface they grow small, thin shells and are called "spat". Oysters prefer warmer water and often prefer reduced salinities as well (brackish water). In those conditions they commonly form reefs from the piled-up shells.

Oysters like other bivalves can form pearls when an irritant (e.g., sand grain) gets inside and can't be expelled. The particle is surrounded by the tissues of the mantle (muscular tissues in contact with the shell), which secrete the same calcium carbonate (CaCO3 / aragonite) mother-of-pearl material as the interior of the shell. Layer upon layer of shell material builds up around the irritant until it is smooth and gets large enough to be considered a pearl. "Cultured" pearls are formed by a bit of human intervention. A pre-formed sphere of shell material is introduced into the mantle and the oyster deposits a thin layer on the surface.

The anatomy of an oyster includes an inhalent and exhalent "siphon", which are the tubes that bring water into the shell and expell it for filter feeding. The gills simultaneously serve the purpose of respiration and feeding. The gills are coated with cilia and mucus that trap food particles. The mucus flows towards the mouth where there is a fibrous structure (style) that rolls up the mucus and crushes it before it is injested by the mouth.

The shell is held together by a combination of a hinge structure with a grooved articulating surface and cylindrical adductor muscle that pulls the two shells together when disturbed. A springy ligament structure in the hinge pushes the shell open when the adductor muscles relax.

Oysters can be eaten steamed, deep fried, smoked, or even raw. You do have to be careful that they haven't filtered any concentrated any nasty algal toxins from the water, or you risk paralytic shellfish poisoning, which is usually caused either by dinoflagellate or diatom blooms of certain types. This is why some oyster fisheries are closed from time-to-time.