In comparisons, fewer applies to individual countable items (a plural count noun), whereas less applies ro general amounts (a noncount noun, always singular).  Fewer refers to number; less refers to quantity.

We have fewer snowflakes and less snow; we have fewer arguments and less anger; we have fewer tears and less sorrow; we have fewer prejudices and less intolerance.

Sense is the key, for some words may go both ways.  For example, we have fewer troubles (in the sense of specific instances of trouble) or less trouble (in the sense of sense of the overall concept of having difficulty).  We may have fewer ambitions (in the sense of specific individual goals) or less ambition (in the sense of the quality referred to as ambition).  We may have fewer fears (where fears means "things that cause us fright") or less fear (where fear means "the condition of being afraid"). 

It is worth noting also that, when we say that fewer refers to countable items and less refers to nouncountable items, we are using "countable" and "noncountable" in the grammatical sense.  For instance, money is countable, but the word money is not usually used as a countable noun – we do not say, "one money, two moneys (or monies)."  Thus, we speak of "less money," not "fewer money."  On the other hand, we do count "one penny or one dollar, two pennies or two dollars."  Thus, the correct comparative term is fewer pennies or fewer dollars (but see the next paragraph).

A further consideration is that we may use countable nouns to represent quantitave measurements – particularly with regard to distances, periods of time, and sums of money.  Although miles, minutes, or dollars (for example) are "countable," we still say, "less than five miles to go," "less than five minutes left," and "less than five dollars."  We are referring to quantities, not countable units.

Finally, we must observe that colloquial, informal usage often breaks this rule.  It is not uncommon to hear people say that "less people voted in this election than in the last" when they should, according to traditional rules, say that "fewer people voted in this election than in the last."  "People" is the plural form of the count noun "person" (one person, two people, etc.).  Although "less people" may be accepted colloquially, "fewer people" is what we should use in formal, written English.