The words keys, cays, quays sound the same but have different meanings and spellings. Why do keys, cays, quays sound the same even though they are completely different words?
The answer is simple: keys, cays, quays are homophones of the English language.
Plural form of cat.
Plural form of category.
Third-person singular simple present indicative form of cat.
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.
Plural form of quay.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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.