tough / tuff are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
tough: (adjective) not given to gentleness or sentimentality. (adjective) physically toughened. (adjective) resistant to cutting or chewing.
tuff: (noun) hard volcanic rock composed of compacted volcanic ash.
tough: t·AH·f
tuff: t·AH·f
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)