We use the quantifiers less and fewer to talk about quantities, amounts and degree. Less and fewer are comparative words.
Less is the comparative form of little. Fewer is the comparative form of few.
Less and fewer with a noun
We usually use less with uncountable nouns. We use fewer with plural nouns:
I do less work at weekends than I used to.
Better cycle routes would mean fewer cars and fewer accidents.
You will often hear less used with plural countable nouns in informal spoken situations, but traditionally it is not considered to be correct:
We’ve got less pizzas than we need. There’s ten people and only eight pizzas. (traditionally correct usage: fewer pizzas)
Less and fewer with of
When we use fewer or less before articles (a/an, the), demonstratives (this, that), possessives (my, your) or pronouns (him, them), we need to use of. We use less of with singular nouns and fewer of with plural nouns:
It was funny to begin with, but as time went on, it became less of a joke.
In ten years’ time, more and more people will be demanding information twenty-four hours a day, from all parts of the world. Fewer of them will be getting that information from newspapers which arrive hours after the news has occurred.
Less and fewer without a noun
We can leave out the noun when it is obvious:
Every year in Britain about 5,000 people die on the roads. Fewer are killed at work. (fewer people)