The difference between less and fewer, like that between lay and lie, is on the brink of extinction, but enough careful speakers and writers observe the difference to make it worth our attention.
In standard usage, less is used to describe amounts:
He has less money than Bill Gates.
They have less soup than we.
Fewer is used to describe things that can be counted:
We have fewer problems now than before.
Fewer voters turned out this year than last.
Here are examples of nonstandard usage in the media:
Maybe Ben and Jerry’s will have less choices. (Fewer choices; less choice)
Their company had less sales. (fewer sales)
Cars that use less gas will result in less emissions. (Less gas, fewer emissions)
Less dollars will be generated. (fewer dollars)
Neutragena: less wrinkles (fewer wrinkles)
Slimfast: more protein, less calories (fewer calories)
What seems to be an exception needs to be noted. One poor NPR reporter came up with this sentence: There is one fewer candidate in the race. I give her points for trying to observe the rule, but the result is not idiomatic. “Less” trumps “fewer” here. Although “candidates” can be counted, one candidate cannot be broken down and counted. There is one less candidate in the race.
Similar to the difference between less and fewer is that between amount and number.
Here is a reporter updating a forest fire:
There’s been no growth in the amount of acres burned. “Acres” can be counted, hence: the number of acres burned. One can talk about the amount of water that has been poured on the fire.
Only time will tell if these distinctions will survive. If enough writers who find them useful use them in their writing, they may make it.