less and fewer

The difference

Less is the comparative of little (used especially before uncountable nouns). Fewer is the comparative of few (used before plural nouns). Compare:

• I earn less money than a cleaner.
• I’ve got fewer problems than I used to have.

In an informal style, less is quite common before plural nouns. Some people consider this incorrect.

• I’ve got less problems than I used to have.

less/fewer with and without of

Before another determiner (article, possessive, etc) or a pronoun, we use less of and fewer of. Compare:

• If you want to lose weight, eat less food. (not … less of food.)
  Fewer people make their own bread these days. (not Fewer of people …)
• I’d like to spend less of my time answering emails.
  At the college reunions, there are fewer of us each year.

less and fewer without nouns

Nouns can be dropped after less and fewer if the meaning is clear.

• Some people go to church, but less/fewer than 20 years ago.

Less can be used as an adverb (the opposite of the adverb more).

• I worry less than I used to.

note

For lesser, (see here).