Many writers and speakers confuse ‘fewer’ and ‘less’, but the practical difference between them is small; everyone understands what you mean if you say ‘less’ when it should be ‘fewer’, and vice versa. Nevertheless, the words have a subtle difference in meaning.
‘Fewer’ is used with reference to discrete concepts – things that are countable and in the plural. For example:
I spend fewer hours cycling each week than you.
There are fewer grains of sand on my feet than on yours.
‘Less’ is used for continuous concepts – meaning things that are essentially uncountable, and singular:
I spend less time cycling each week than you.
There is less sand on my feet than on yours.
In the four sentences above, ‘hours’ is plural but time is singular, likewise ‘grains of sand’ and ‘sand’.
‘Less’ is also used with quantities when the individual units of measurement aren’t the focus, as in:
Brian drank less than two teaspoons of milk.
Brian is less than five feet tall.
Substitute ‘fewer’ for ‘less’ in the two sentences above; this would mean Brian drank either one or no teaspoonfuls of milk (as opposed to half or 1.7) and is exactly zero, one, two, three or four feet tall (as opposed to four feet and seven inches) – feasible but not very likely intended meanings.