Scallops are bivalves (mollusks with two hinged shells). Although there are hundreds of species of scallops in the world's oceans and bays, only a handful are commonly available. The largest and most popular are sea scallops, primarily harvested in the Atlantic from Eastern Canada to North Carolina, but also from Peru, Japan, and Russia.
Size can vary greatly, but generally there are about 20 scallops to a pound.
Ask for dry sea scallops, which means that they haven't been soaked in a sodium solution. The solution whitens and plumps the scallops, but when you cook them, all that liquid leaches out, making it impossible to achieve a good sear. Fresh scallops should appear moist but not milky. Refuse any that have a feathery white surface (a sign of freezer burn) or dried and darkened edges (a sign of age). Always ask to smell scallops before buying. They should smell somewhat briny and seaweedy, but not offensive, sharp, or at all like iodine. If the scallops have no smell and a uniform stark-white color, chances are they've been soaked in the sodium solution.
Remove the tough abductor muscle from the side of each scallop (some scallops are sold with the muscle already removed). If you feel any grit on the scallops, rinse them under cold water (otherwise, avoid rinsing, which can wash away flavor). Pat the scallops dry with paper towels; surface moisture impedes browning.
Store scallops in the coldest part of your refrigerator. Ideally, nest the bag or container in a larger bowl of ice to ensure that they really stay cold. But avoid direct contact with ice—it will leach flavor and deteriorate the texture of the scallops.