Less v/s Fewer

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".)