A quay is a platform to which ships are docked for loading and unloading. A traditional quay runs parallel to the shore, and is sometimes simply made by carving into and reinforcing the shore, creating a sort of natural docking point. In contrast, a wharf runs perpendicular to the shore, creating the potential for much more docking and unloading space; most major harbors today use wharves, with quays appearing in smaller, private harbors.
Quays can be built from a number of materials. In ancient times, they were built right into the banks of a river or bay, and heavily reinforced with stone. A modern quay may be built from stone, concrete, wood, or metal, with post and pier construction being common, as it is much cheaper than solid construction. A quay may be very small, or long enough to accommodate several ships, and warehouses are often situated conveniently nearby.
Historically, a quay would have been a place of high activity, bordered by warehouses, shipping offices, and the homes of some prominent harbor officials and crewmembers. The surrounding environment might have been a bit unsavory, thanks to facilities established to cater to the needs of sailors, and in fact the areas around many docks and shipyards continue to be a bit infamous for their often blighted and dulled appearance. In the modern era, where communication methods are more varied, it is not longer necessary to live close to the docks to run a shipping business, and as a result dockyards have been largely taken over by warehouse space.
If you're puzzling over how to pronounce this word, the correct pronunciation is “key,” and the word is in fact a variant spelling on “key,” a Middle English word which dates to the early 1300s in the sense of a docking place. To add to the general confusion, the word “key” is also used to refer to small islands such as those found in abundance in the Caribbean, perhaps because such islands represented convenient docking places for boats and ships in the age of exploration.
Access to a quay is typically limited to reduce confusion and the risk of injury. Dockhands and other shipyard workers may be present on a quay, along with the crews of ships arriving or departing at the quay. If passengers are being accepted on a ship docked at a quay, they may be isolated to a specific area to ensure that they are not lost or hurt, as a busy quay can be a confusing and intimidating place for people unfamiliar with the environment.