The words tartar, tarter sound the same but have different meanings and spellings. Why do tartar, tarter sound the same even though they are completely different words?
The answer is simple: tartar, tarter are homophones of the English language.
Dentistry A hard yellowish deposit on the teeth, consisting of organic secretions and food particles deposited in various salts, such as calcium carbonate. Also called calculus.
A reddish acid compound, chiefly potassium bitartrate, found in the juice of grapes and deposited on the sides of casks during winemaking.
comparative form of tart: more tart
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License 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").