Via Doug Powers at MichelleMalkin.com, I see Obama has declared that the Republican plan is
"let's have dirtier air, dirtier water, less people with health insurance."
He means fewer people with health insurance. (And if the people who don't get health insurance are illegal immigrant, I'm for it!)
But seriously, this is a major point of English usage. If you're a native speaker of English, you can probably distinguish between less and fewer most of the time without thinking about it. No one says "fewer money" although they do occasionally say "less dollars", which is the same error Obama is making here.
Less is for mass quantities, things you don't count individually, and fewer is for things you do count one by one.
The technical name for this is "count" and "non-count" nouns,
By a strong effort of will, I have made this post about nothing but the grammatical point.