[This is a fast-access FAQ excerpt.]
The rule usually encountered is: use "fewer" for things you
count (individually), and "less" for things you measure: "fewer
apples", "less water". Since "less" is also used as an adverb
("less successful"), "fewer" helps to distinguish "fewer successful
professionals" (fewer professionals who are successful) from "less
successful professionals" (professionals who are less successful).
(No such distinction is possible with "more", which serves as the
antonym of both "less" and "fewer".)
"Less" has been used in the sense of "fewer" since the time of
King Alfred the Great (9th century), and is still common in that
sense, especially informally in the U.S.; but in British English it
became so rare that the 1st edition of the OED (in a section
prepared in 1902) gave no citation more recent than 1579 and gave
the usage label "Now regarded as incorrect." The 2nd edition of the
OED added two 19th-century citations, and changed the usage label to
"Frequently found but generally regarded as incorrect."
Fowler mentioned it only in passing, and cited no real examples.
In a section whose main intent was to disparage "less" in the sense
"smaller" or "lower", he wrote: "It is true that less and
lesser were once ordinary comparatives of little [...] and that
therefore they were roughly equivalent in sense to our smaller
[...]. The modern tendency is so to restrict less that it means
not smaller, but a smaller amount of, is the comparative rather
of a little than of little, and is consequently applied only to
things that are measured by amount and not by size or quality or
number, nouns with which much and little, not great and
small, nor high and low, nor many and few, are the
appropriate contrasted epithets: less butter, courage; but a
smaller army, table; a lower price, degree; fewer opportunities,
people. Plurals, and singulars with a or an, will naturally
not take less; less tonnage, but fewer ships; less manpower,
but fewer men [...]; though a few plurals like clothes and
troops, really equivalent to singulars of indefinite amount, are
exceptions: could do with less troops or clothes."
Webster's New International Dictionary, 2nd ed. (1934), gave the
usage label "now incorrect, according to strict usage, except with a
collective; as, to wear less clothes." Of the panelists for The
Harper Dictionary of Contemporary Usage (1975), 76% said that they
observed "less"/"fewer" distinction in speech, and 85% in writing.
The editors noted: "even those panelists who have not observed the
distinction in the past now regard it as a useful precept to bear in
mind in the future."
Partisans of "fewer" use "one car fewer" rather than "one
fewer car", and "far fewer" rather than "much fewer".