Both of these words imply a comparison with something larger in number or amount. Fewer is preferred when number is involved (fewer houses on this street, fewer fish in the stream). Less is used in several ways: it is applied to material in bulk (less sugar in the coffee); with abstractions (less honor in business dealings); with matters involving degree and value (1 is less than 2). Although many writers and speakers use these words interchangeably, fewer should be used to refer only to numbers or to units capable of being counted: "The less money we have, the fewer supplies we can bring."
Dictionary of problem words and expressions. Harry Shaw. 1975.