Pound-for-pound, the most popular seafood in the US is shrimp - frozen shrimp. Properly packaged, shrimp or prawns can be stored for several months in cold storages and home freezers without losing quality. Frozen shrimp are extremely versatile, and no kitchen is complete without a frozen 'stash' of this popular ingredient.
IQF (or, Individually Quick Frozen) shrimp thaw quickly, and it's easy to grab just the amount you need from your freezer. Most (frozen) shrimp sold by FishEx are IQF.
Block-frozen shrimp are also common. Shrimp in this form are popular with commercial chefs because the blocks have an extremely long shelf life, they are compact, and they thaw reasonably quickly. Many of the imported tiger shrimp from Asia arrive in block form. Because shrimp with shells are not vacuum-sealable (the shells have spikes which puncture the vacuum bags), frozen shrimp are almost always glazed, or lighty coated with ice - which protects from freezer-burn.
Tip: Take the term "fresh-frozen shrimp" with a grain of salt. While the term implies quality - and that the shrimp were frozen at the peak of "freshness" - the phrase doesn't actually have a lot of meaning. It's always best to buy frozen shrimp from a vendor that you trust.