Scallop, interestingly, has several food-related meanings; I'll deal with the (eyed) bivalve mollusk first, and then speak briefly about the others.
Bivalve refers to the fact that the scallop has two shells, like its cousins the oyster and the clam, but the scallop shell is distinguished by its attractiveness. Scallop shells are lovely fan-shaped affairs, often used as serving dishes for Coquilles St. Jacques. Mollusk is one of the two main culinary classifications of shellfish, the other being crustacean.
In North America we tend to eat only the adductor muscle of the scallop, the part that hinges the two shells together, but all parts of the scallop (except the shell) are in fact edible, including the roe. (What we don't eat many Europeans relish.) In North America, at least, the many species of scallops are generally classified into two groups, bay and sea. Bay scallops come from the east coast and are tiny (perhaps 1/2" in diameter), sweet, and succulent - and the more expensive. They are in season in the fall. Sea scallops are about 1-1/2" in diameter, chewier, and less expensive, though still sweet and moist. They are in season from midfall to midspring.
Whichever type you buy, they should have a sweet smell, moist sheen, and sticky feeling if you touch them. Scallops vary in colour from ivory to pinkish beige to tan, or even slightly grey, but should never be stark white; if they are, they've been soaked in a water and phosphate mixture to increase their weight and make them last longer. These soaked scallops taste inferior, and they don't brown properly, but sadly, the practice of soaking is the norm, so look for a reputable fishmonger who sells lovely coloured scallops sitting in only a little, if any, liquid, and shun the one that sells white scallops in a pool of milky juices.
Just as crab is often not crab at all, but rather haddock or pollock that bears no real resemblance to true crabmeat, unscrupulous fishmongers and restaurateurs are apparently substituting skate or shark meat for scallops, using a cookie cutter to stamp out scallop-shaped pieces of seafood. If you find a scallop which has a grainy, striated texture, you've probably got a faux scallop. I advise you to march up to the cad, cry "For shame!", and take your scallop business elsewhere in the future.
And what of the other culinary scallops?
Scallop also refers to a thin, boneless, round or oval slice of meat or fish. It's usually breaded and quickly sauteed. Think veal scaloppine or escalope.
To scallop means to layers slices of a food with cream sauce and bake it in a casserole, usually topped with crumbs: scalloped potatoes, my childhood favourite.
Finally, to scallop can also mean to form a decorative edge which is crimped or fluted, as on the raised rim of a pie crust.