Less vs fewer

People sometimes confuse less with fewer. Particularly with less, the common mistake is to use it when fewer is the correct word:

(1) ?We had less customers yesterday.

Though both words mean not as many, they are used differently. The general rule is to use less for uncountable nouns and fewer for countable nouns:

Less for uncountable nouns

(2) There is less traffic now.
(3) I listen to less music when I'm travelling.
(4) There's less smoke here.
(5) I drank less coffee yesterday.

Fewer for countable nouns

(6) She made fewer calls last week.
(7) I had fewer coins than you.
(8) Fewer than 50 people attended the talk.
(9) There are fewer mistakes in her essay.

However, with nouns that denote time, money and distance, we use less:

(10) He had less than three hours to make a decision.
(11) I had less than ten dollars in my pocket.
(12) She ran less than five kilometres.