In many cases there is no difference.
Example: Less than five people came to the party. = Fewer than five people came to the party
but here is a case where there is a difference:
Five less two equals three. (correct)
Five fewer two equals three. (incorrect)
The word "less" can be used in more ways than the word "fewer".
Less can be a noun, verb, adverb, adjective or pronoun.
But the word "fewer" can only be an adjective or a pronoun.
Consult a dictionary to see the different usages.