Pier vs. quay
Basically, the two terms are quite similar and can be easily interchangeable, but we can draw some differences:
A pier is a platform (typically a wooden structure) extending from a shore over navigable water and supported by piles or pillars, for use as landing place (providing access to ships or boats), promenade, etc.
A quay is also a landing place (made of concrete, stone, wood, etc.) usually built parallel to the shoreline and beside navigable water at a harbour or river for the loading and unloading of ships.