Less and fewer are easy to mix up. They mean the same thing—the opposite of more—but you use them in different circumstances (1). The basic rule is that you use less with mass nouns and fewer with count nouns.
Count Nouns Versus Mass Nouns
Now I'm worried that I've scared you off, but it's easy to remember the difference between mass nouns and count nouns.
A count noun is just something you can count. I'm looking at my desk and I see books, pens, and M&M's. I can count all those things, so they are count nouns and the right word to use is fewer. I should eat fewer M&M's.
Mass nouns are just things that you can't count individually. Again, on my desk I see tape and clutter. These things can't be counted individually, so the right word to use is less. If I had less clutter, my desk would be cleaner. Another clue is that you don't make mass nouns plural: I would never say I have clutters on my desk or that I need more tapes to hold my book covers together.
Sometimes it isn't obvious if something is a mass noun or a count noun because some words can be used in different ways. For example, coffee can refer to either a mass of liquid or a cup of liquid. If you're responsible for filling the coffee decanter at a wedding, and you're getting carried away, your boss might ask you to make less coffee. But if you're a waiter serving cups of coffee to the tables, and the crowd is waning, your boss might tell you to bring out fewer coffees next time. She means cups of coffee, but it's common to hear that shortened to just coffee as in “Bring me a coffee, please.” Remember that I said mass nouns (like coffee) can't be made plural? In this example, I've made a mass noun plural, but in the process I transformed it into a count noun. So the rule still holds.
Furniture is another tricky word; it isn't immediately obvious whether it is a mass noun or a count noun. If I think of a furniture store, I think of lots of individual pieces of furniture, but furniture is a collective name for a mass of stuff. You could say, “Look at all those couches,” but you would never say, “Look at all those furnitures.” Furniture is a mass noun. Therefore, you'd say, “We need less furniture in this dance hall. Can we have fewer chairs?”
Exceptions
There are exceptions to these rules; for example, it is customary to use the word less to describe time, money, and distance (2, 3). For example, you could say, “That wedding reception lasted less than two hours. I hope they paid the band less than $400.” So keep in mind that time, money, and distance are different, but if you stick with the quick and dirty tip that less is for mass nouns and fewer is for count nouns, you'll be right most of the time.
Memory Tricks
There are two ways that I remember when to use less and when to use fewer.
First, I think of the classic example of the grocery store express lane. Most of the signs for these lanes read, “10 items or less,” and that's just wrong. The signs should read, “10 items or fewer,” because items are individual, countable things. Between hearing people complain about the signs and seeing the signs every week or so, it sticks in my head that it should be fewer items. And when I stand in line and count the 15 items that belong to the person in front of me in the 10-items-or-fewer lane, I'm strongly reinforcing the idea that items are countable.
Second, I have a memory trick. I think of Aardvark sitting by a lake. He's fishing. The water is low in the lake this year, so there is less water in the lake. Less and lake both begin with the letter l. There is less water in the lake. Squiggly is worried about dinner. Aardvark usually catches four fish, but what if there are only three? “We'll have fewer fish for dinner,” Squiggly thinks to himself fretfully. Fewer and fish both start with the letter f, and Squiggly is counting fish in his head. We'll have fewer fish for dinner.
