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."
