The words yack, yak sound the same but have different meanings and spellings. Why do yack, yak sound the same even though they are completely different words?
The answer is simple: yack, yak are homophones of the English language.
Slang Variant of yak2.
A wild, shaggy-haired ox (Bos grunniens) of the mountains of central Asia.
A domesticated yak, used as a work animal or raised for meat and milk.
To talk persistently and meaninglessly; chatter.
Prolonged, sometimes senseless talk; chatter.
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").