The words tough, tuff sound the same but have different meanings and spellings. Why do tough, tuff sound the same even though they are completely different words?
The answer is simple: tough, tuff are homophones of the English language.
Able to withstand great strain without tearing or breaking; strong and resilient: a tough all-weather fabric.
Hard to cut or chew: tough meat.
Physically hardy; rugged: tough mountaineers; a tough cop.
Severe; harsh: a tough winter.
A rock composed of compacted volcanic ash varying in size from fine sand to coarse gravel. Also called tufa.
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").