This article claims that the Guardian style guide was unhelpful. Possibly he should have read it:
"fewer/less: fewer means smaller in number, eg fewer coins; less means smaller in quantity, eg less money"
Simply put fewer is for things which can be counted, less is for things which can not. So you have fewer cylinders of air, but less air. If you can say "one X" and have it make sense you should use fewer, otherwise use less.
That's the rule as it's meant to work, the more interesting question is "does the rule matter any more?", to which I'd answer probably not.
You don't see supermarkets with "5 items of fewer" lanes and most people don't really see any difference between the two terms. The reality is that language continues to evolve, as it has always done, and the distinction is really now just one for pedants (guilty as charged). If you need proof of this then the just look at the figures in quoted blog post (assuming they're even vaguely right), which suggest that most Guardian journalists are ignoring their own style guide and their sub-editors (a job title which pretty much comes with pedantry in the job spec) are letting them.
