As with all fish, you get a different answer depending on whom you ask. Alan Davidson, author of The Oxford Companion to Food and one of the world's foremost authorities on seafood, refers to the sand dab as a European fish, found in the North Atlantic. It is a flatfish, with a brown back, and can reach 16 inches in length, although is often closer to 10. He says it is "a good fish, with a pleasing flavour, well suited to being fried."
Davidson says that English colonists conferred the name dab on other species that they found in their travels that appeared similar to the original dab. So a dab becomes a lot less specific on this side of the Atlantic. There is a species of sand dab found in the Pacific Ocean from California to Alaska, with a market weight of 4 to 12 ounces. There is another flatfish found in the Atlantic, with a market weight of 2 to 3 pounds, that is known as the sand dab, but is more correctly named American plaice. The plaice has also been called a flounder and a sole, although Davidson says its scientific name (hypoglossoides platessoides) suggests it is a relative of the halibut. So you are likely to find various varieties of flatfish being referred to a dabs or sand dabs.
Dabs caught in the spring and those that lived on a sandy seabed (as opposed to mud) are said to have the best flavor. They are best poached, fried, or grilled.