Fewer vs Less
Quick answer: Use fewer for things you can count one by one (fewer cookies); use less for quantities you measure (less sugar, less time). The express-lane sign '10 items or less' is the famous exception everyone argues about.
Fewer
Determiner: a smaller number of countable things.
“There were fewer students in class today.”
Less
Determiner: a smaller amount of an uncountable quantity.
“This recipe needs less butter.”
How to remember it
If you can count them, count on fewer.
Common mistake
'Less people showed up' — people are countable: fewer people.