Traditionally, fewer, a plural noun, has most often been used to refer to individual units that can be counted
Ex: There are fewer buttons on this shirt. No fewer than forty of the fifty voters supported the measure.
The band's most recent concert drew fewer people than usual.
Less, a singular noun, is used to refer to uncountable quantities such as water or patience. Use "less" for bulk quantities, time, or uncountable quantities.
Ex: She eats less every day. I have less patience than I used to.
I drank less water than she did.
Examples of fewer:
He worked fewer hours than I did.
"Fewer than 10 applicants applied for the job."
Examples of Less
This coat costs less than ten dollars.
He won by less than five percent of the vote.
He would settle for no less than 3.5 gallons of the highest-grade milk.