less and fewer

Less is the comparative of little (used especially before uncountable nouns).
Example:
I earn less money than a clerk.

Fewer is the comparative of few (used before plural nouns).
Example:
I’ve got fewer friends now than I used to have.

Less of and fewer of are used before determiners (like the, my or this) and pronouns.
Example:
I’d like to spend less of my time answering calls from strangers.

Before nouns without determiners, of is not used.
Example:
If you want to lose weight, eat less food.

Lesser is used in a few expressions to mean ‘smaller’ or ‘not so much.’
Example:
the lesser of two evils
a lesser-known writer