1). Less and Fewer -- Difference -
Less is the comparative of little ( used especially before uncountable nouns).
e.g - I earn less money than you.
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
5) Use less when referring to statistical or numerical expressions.
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