Less vs Fewer

1). Less and Fewer -- Difference -

Less is the comparative of little ( used especially before uncountable nouns).

e.g - I earn less money than you.

Whereas

Fewer is the comparative of few ( used before plural nouns)

e.g - I've got fewer problems than I used to have.

2). Less of and fewer of - used before determiners such as the,, my, this and before pronouns.

e.g - At the college reunions, there are fewer of us each year.
I'd like to spend less of my time answering mails.

* Before nouns without determiners, of is not used.

e.g - If you want to lose weight, eat less food. (NOT less of food)

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

e.g - Some people go to church, but less/fewer than 20 years ago

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

e.g - I worry less than I used to.

4). Lesser - used to mean "smaller" or "not so much"

e.g - the lesser of two evils.

GMAT Examples - Sentence No 49 at the link below
http://gmatsentencecorrection.blogspot.com


5) Use less when referring to statistical or numerical expressions.
e.g:
Sara is less than five feet tall
Your issue essay should be a thousand words or less

NOTE: It's possible to regard the quantities as sums of countable measures