The words tongue, tung sound the same but have different meanings and spellings. Why do tongue, tung sound the same even though they are completely different words?
The answer is simple: tongue, tung are homophones of the English language.
The fleshy, movable, muscular organ, attached in most vertebrates to the floor of the mouth, that is the principal organ of taste, an aid in chewing and swallowing, and, in humans, an important organ of speech.
An analogous organ or part in invertebrate animals, as in certain insects or mollusks.
The tongue of an animal, such as a cow, used as food.
A spoken language or dialect.
simplified, and former, spelling of tongue
A tung tree.
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").