Have you ever gone to the supermarket and been lured into the express checkout aisle because you have Twelve Items or Less? Strictly speaking, it turns out that sign confuses less with fewer.
The words less and fewer both apply to quantities, but the word less typically refers to things that cannot be counted, such as emotions, abstract notions of time or unmeasured amounts of water. For example:
The drought means there is less water in the creek this year.
The word fewer refers to quantities that can be counted. For example,
Frank had less anxiety about the exam after doing some extra studying.
There were fewer people at the game last night than there were a week ago.
Following this rule and returning to the placard at the supermarket, it would therefore read, "Twelve items or fewer." The intended result of the express aisle is that you'll spend less time waiting.
On Mondays, there are always fewer customers in the restaurant.
It's a fine distinction, but when it comes to formal writing or speech, it's best to use fewer for things we can count, and less for things we can't.
Sources: Oxford Dictionary of Current English and The
Elements of Style by William Strunk, Jr. and E.B. White.