Brachiopods, also known as lampshells or "brachs," are a group of shellfish, members of phylum Brachiopoda. Although some of them look superficially like clams, they're really completely different — both on the outside and the inside. Brachiopods are phylogenetically unique enough to merit their own phylum, whereas clams are bivalves, part of phylum Mollusca. Brachiopods are called articulate or inarticulate based on whether their two shells have a hinge (articulate) or are held together based on muscles alone (inarticulate).
Brachiopods are stationary filter feeders that feed by means of a frilled appendage called a lophophore. Unlike bivalves, which have two symmetrical shells on either side, brachiopods have shells symmetrical about a central axis but not with respect to one another. Part of this has to do with the brachiopods' other appendage — the pedicle. The pedicle is a fleshy stalk that the brachiopod uses to affix itself to a surface, usually permanently. One shell has a special opening for the pedicle to protrude, hence it is called the pedicle valve. The pedicle valve is the top shell of the animal, and it is often thicker and harder than the lower valve (the brachial valve), to protect against attacks from above.
Though rare today, brachiopods are very important in the context of evolution, for several reasons. Brachiopods were the first bilaterian animals to enclose themselves in a hard shell and adopt a stationary lifestyle. Brachiopods dominated the ocean bottoms for the entire Paleozoic era (from 542 to 251 million years ago), far outnumbering the bivalves that cover the ocean floor today. Because of their abundance and hard shell, brachiopods are also the most common fossil. They were especially successful from the Ordovician period onwards, when many filter feeders evolved and began to exploit the abundant food particles building up in the Earth's oceans.
During the Permian-Triassic extinction 251 million years ago, brachiopods all but went extinct. 96% of brachiopod species died off. Since then, brachiopods have been outcompeted by bivalves, which use siphons instead of a lophophore to feed, and are often mobile, crawling along the sea floor or burrowing into the substrate using a muscular foot.