The words knob, nob sound the same but have different meanings and spellings. Why do knob, nob sound the same even though they are completely different words?
The answer is simple: knob, nob are homophones of the English language.
A rounded control switch or dial.
A rounded protuberance.
A prominent rounded hill or mountain.
A rounded handle, as on a drawer or door.
Slang The human head.
The jack of the same suit as the card turned up by the dealer in cribbage, scoring one point for the holder.
Chiefly British Slang A person of wealth or social standing: "The nobs were forever snubbing the snobs” ( Conor Cruise O'Brien).
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").