FEWER, LESS
The general rule is to use less for quantity and fewer for number. Thus: "The building has less floor space than the Empire State, yet it contains no fewer than 1,200 offices."

There is one oddity about fewer: Whereas it is fine to write, "The Liberals won three fewer seats than in the previous election," you run into idiom trouble if you reduce the number to one; you cannot say "one fewer seats," nor can you say "one fewer seat." The only escape hatch is "one seat fewer."

The other problem about fewer is to distinguish whether it is quantity or number that is being spoken of. For instance: "Not many of these buildings are fewer than thirty years old." The thought here is not of individual years but of a period of time; therefore, less. Another example: "Some professors earn fewer than $7,500* a year." Make it less. The thought is not of separate dollars but of a sum of money.