There seems to be confusion about when to use the word less and when to use the word fewer. Fewer should be used when you are talking about things that can be counted. Grammar Girl calls them “count nouns.”
- He took three pencils and left fewer than four on her desk.
Less is used when you are talking about things that cannot necessarily be individually counted. Grammar Girl calls those “mass nouns.”
- If he used less sarcasm, he might have more friends.
Of course, we are talking about the English language, so there are exceptions. The word less is typically used for measurements of time, money, and distance.
- He had less than four hours of work left before his vacation.
An interesting fact is that the signs in the grocery store for “10 items or less” is actually grammatically incorrect because you can count the items you put on the grocery belt (count nouns). To be grammatically correct, it should be “10 items or fewer.” That is one way to remember the difference (if remembering horrible mistakes helps you remember how it really should be). There is a belief, however, that in less formal writing, “10 items or less” sounds less stuffy, so is appropriate to use. Working for lawyers, however, has trained me that no writing is less formal, so I’m sticking with the “rule” and believing that all grocery stores are wrong! 
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