The words boy, buoy sound the same but have different meanings and spellings. Why do boy, buoy sound the same even though they are completely different words?
The answer is simple: boy, buoy are homophones of the English language.
A male child.
A son: his youngest boy.
Often Offensive A man, especially a young man.
Informal A man socializing in a group of men: a night out with the boys.
A float moored in water to mark a location, warn of danger, or indicate a navigational channel.
A life buoy.
To keep afloat or aloft: a glider buoyed by air currents.
To maintain at a high level; support: "the persistent ... takeover speculation, which has buoyed up the shares of banks” ( Financial Times).
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").