The words quay, key sound the same but have different meanings and spellings. Why do quay, key sound the same even though they are completely different words?
The answer is simple: quay, key are homophones of the English language.
A notched and grooved, usually metal implement that is turned to open or close a lock.
A similar device used for opening or winding: the key of a clock; a can that has a key attached.
A means of access, control, or possession.
A vital, crucial element.
A wharf or reinforced bank where ships are loaded or unloaded.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").