Writing Tips: Less vs. Fewer

Perhaps the most predominant misuse of the word less occurs in grocery stores, in the popular express lanes allowing your use of the lane as long as the items you have do not exceed a certain number. If you're like me, and I know you are, it takes all your strength to put your Cocoa Puffs on the checkout counter under the sign that reads "7 items or less" without telling someone-anyone-that "less" should be "fewer." Luckily for me the grocery store at which I shop most frequently has grammatically correct express-lane signs. Cheers to you Wegmans.

However, when I'm in other grocery stores that use "less," I fight the urge to point out this mistake to my line mates and cashier, not because I don't want to sound like a know-it-all, but because I know that every person within earshot would want to immediately make out with me, and, well, I'm married. Because I am just that hot, it is actually one of the first things I look for/notice in a new grocery store. In May I took my youth group on a mission trip to the Bowery Mission in New York City, and we stopped in at Whole Foods on Bowery. I looked, I saw, I smiled: the sign said "fewer." Cheers to you too Whole Foods.

Now for the lesson. Both less and fewer mean the opposite of more, so it's easy to confuse them, but they are used in different instances: less for "not as much," and fewer for "not as many." (I know-much, many, what's the difference? Read on.) Less is used with mass nouns, and fewer is used with count nouns.

What is a mass noun? A plural entity that cannot be counted. "Much" cannot be counted.
What is a count noun? A plural entity that can be counted. "Many" can be counted.

So, if you can count it (glasses, blankets, crackers, cheese cubes) use fewer.
If you cannot count it (water, love, sun, wine), use less.

Example:
The second beach we went to had less sand and fewer lifeguards.
The small cart had less space than the large cart, so I put in fewer items.

Exceptions (because you know they exist): less is used with money, distance, and time.

Jack Lynch, an associate professor of English at Rutgers University has an online style guide, and discusses this topic here.
And of course I recommend Grammar Girl's explanation.

And you're welcome.