Less is used with uncountable nouns, but fewer is used with plural countable nouns.
I have fewer beer than he does.
(uncountable noun/beer)
I have less beer than he does.I did fewer work than last week.
I did less work than last week.Ten items or less.
(plural countable noun/items)
Ten items or fewer.There are less people in the lesson today.
There are fewer people in the lesson today.- I have fewer bananas than you, but you have less juice then me.
