In the latest episode of season five of Game of Thrones, Stannis Baratheon showed that even in the harsh lands of Westeros you can't beat a little bit of good grammar.
In the episode, titled Kill the Boy, when confronted with Othell Yarwyck's statement: "Let them die! Less enemies for us", Baratheon quite rightly quips that it should, in fact, be "fewer enemies".
Good on you, Stannis. Top marks. But why is he right?
The rule can be defined with the popular phrase "fewer cows, less milk", which reminds us that when using what is known as a 'countable noun', in this case 'cows', and there aren't as many as before, then we say 'fewer'. When we have a noun that is deemed 'uncountable', such as milk, and there's not as much as before, it is 'less milk'.
If you throw that milk into bottles, then we have ourselves a countable noun, and it would then of course then be "fewer milk bottles".
If that all still sounds a little confusing then think of it as simply as 'fewer' being used for plurals and 'less' being used for non-plurals and you should avoid being decapitated by angry mythical grammar pedants. Or use the Plain English Campaign's rule: "less means 'not as much'; fewer means 'not as many'". [Zap2It]