FEWER should be used when one can count the items/subjects in question. It is usually used with a following “THAN”.
1) Today I had fewer than ten phone calls.
2) Fewer awards than advertised were distributed at the Honors Ceremony.
LESS is used for items/subjects that cannot be measured.
1) You should work less to be healthy.
2) Since we had less snowfall this year, we decided not buy that snowblower.