Here is the rule as it is usually encountered: fewer refers to number among things that are counted, and less refers to quantities or amount among things that are measured. This rule is simple enough and easy enough to follow. It has only one fault - it is not accurate for all usage. If we were to write the rule from the observatoin of actual usage, it would be the same for fewer: fewer does refer to number among things that are counted. However, it would be different for less: less refers to quantity or amount among things that are measured and to number among things that are counted. Our amended rule describes the actual usage of the past thousand years or so.
As far as we have been able to discover, the received rule originated in 1770 as a comment on less:
This Word is most commonly used in spaking of a Number; where I should think Fewer would do better. No Fewer than a Hundred appears to me not only more elegant than No less than a Hundred, but more strictly proper - Baker 1770
Baker's remarks about fewer express clearly and modestly - "I should think," "appears to me" - his own taste and preference. It is instructive to compare Baker with one of the most recent college handbooks in our collection:
Fewer refers to quantities that can be counted individually... Less is used for collective quantities that are not counted individually... and for abstract characteristics. - Trimmer & McCrimmon 1988
Notice how Baker's preference has here been generalized and elevated to an absolute status, and his notice of contrary usage has been omitted. This approach is quite common in handbooks and schoolbooks; many pedagogues seem reluctant to share the often complicated facts about English with their students.
How Baker's opinion came to be an inviolable rule, we do not know. But we do know that many people believe it is such. Simon 1980, for instance, calls the "less than 50,000 words" he found in a book about Joseph Conrad a "whopping" error.
The OED shows that less has been used of countables since the time of King Alfred the Great - he used it that way in one of his own translations from Latin - more than a thousand years ago (in about 888). So essentially less has been used of countables in English for just about as long as there has been a written English language. After about 900 years Robert Baker opined that fewer might be more elegant and proper. Almost every usage writer since Baker has followed Baker's lead, and generations of English teachers have swelled the chorus. The result seems to be a fairly large number of people who now believe less used of countables to be wrong, though its standardness is easily demonstrated.
In present-day written usage, less is as likley as or more likely than fewer to appear in a few common constructions. One of the most frequent is the less than construction where less is a pronoun. The countables in this construction are often distances, sums of money, units of time, and statistical enumerations, which are often thought of as amounts rahter than numbers. Some examples:
The odometer showed less than ten thousand miiles - E.L. Doctorow, Loon Lake, 1979
... he had somewhat less than a million to his name when he went to Washington - David Hablestam, Harper's, February 1971
I was never in Europe for less than fourteen months at a time - James Thurber, letter, 18 July 1952
Her agency, less than 5 years old, is a smashing success - Donald Robinson, Ladies' Home Jour., January 1971
... an allied people, today less than 50,000 in number - W.B. Lockwood, A Panorama of Indo-European Languages, 1972
"... I've known you less than twenty-four hours..." - Agatha Christie, Who Didn't They Ask Evans?, 1934
Fewer can be used in the same constructions, but it appears less often than less. It is sometimes used in such a way as to make one suspect that an editor rather than a writer is responsible for the fewer:
... has never gained fewer than 1,222 yards in a season - Rick Telander, Sports Illustrated, 5 September, 1984
Some contemporary usage writers concede that this use of less is acceptable.
The [/i]no less than[/i] construction noticed by Baker tends still to have less more often than fewer:
The class of 1974... included no less than 71 new Democrats - Tip O'Neil with William Novak,Man of the House, 1987
It is spoken by no less than 100 million in Bengal and bordering areas - W.B. Lockwood, A Panorama of Indo-European Languages, 1972
Less is the usual choice in the "twenty-five words or less" construction:
... readers are encouraged to keep their comments to 500 words or less - Cjamge, January-February 1971
... of all the millions of families in the country, two out of three consist of only three persons or less - Mark Abrams, London Calling, 9 Oct. 1952
... and now know enough to create little fictions that in 30 seconds or less get right to the heart of desire iself - Mark Crispin Miller, John Hopkins Mag., Winter 1984
Kilpatrick 1984 defends this less and the one just above. Less is also frequent when it follows a number:
... almost $10 million les sthan for 1969 - Annual Report, Borg-Warner Corp., 1970
Many bulls fought in Madrid weigh 100 kilos less - Tex Maule, Sports Illustrated, 29 July 1968
... at thirty-three on my part, and few years less on yours - Lord Byron, letter, 17 Nov. 1821
And of course it follows one:
... one less scholarship - Les A. Schneider, letter to the editor, Change, September 1971
One less reporter - Don Cook, Saturday Rev., 24 Jne 1978
Less is also frequently used to modify ordinary plural count nouns. In present-day English this usage appears to be more common in speech (and reported speech) than it is in discursive writing. It is likely that some of the plural nouns in the examples were thought of as uncountable amounts rather than numbers.
... Goldsmith took less pains than Pope... to create images of luxury in the reader's mind - John Butt, English literature in the Mid-Eighteenth Century, edited & completed by Geoffrey Carnall, 1979
... Americans pay less taxes than most of the inhabitants of developed countries 0 Robert Leckachman, quoted in Center Mag., January-February 1970
The less sodium you consume, the less drugs you're likely to need -0 Jane E. Brody, N.Y. Times, 4 May 1980
You have to make less mistakes - Victor Temkin, quoted in N.Y. Times, 4 May 1980
... lower rates... lazy days, and less crowds - L. Dana Gatilin, Christian Science Monitor, 23 Oct. 1979
Less people exercise their right to vote - William Scranton, quoted in Celebrity, October 1976
Uses such as the above, even those where fewer might have been more elegant, have been standard for more than a millennium. If you are a native speaker, your use of less and fewer can reliably be guided by your ear. If you are not a native speaker, you will find that the simple rule with which we started is a safe guide, except for the constructions for which we have shown less to be preferred.