Both of these words mean the same thing: they're the opposite of more. Their uses are what differentiate them.
- You use less with noncount nouns
- You use fewer with count nouns
Oh, you want to know what those are? Abso-fruitly. You're lucky we love grammar so much.
- Noncount nouns are things you can't count individually, like clutter, traffic, and sand
How would you like the task of sifting through every grain of sand on the beach? Not very much, we would guess. That's why we just don't bother to count 'em.
Relatedly, you can't make noncount nouns plural. We dare you to try. (And "the beautiful sands of Cabo" doesn't count.)
Want to take a stab at defining count nouns?
- Count nouns are things you can count.
You see three cupcakes, and there are three of you at the table. If there were any fewer cupcakes, someone would have been really unhappy.
Though these definitions are easy to remember, some words can be tricky, so try this exception to the rule on for size:
Even though we count hours, dollars, and miles, you use the expression less than to describe time, money, and distance.
Example
Maybe if I took less AP classes I would have fewer stress-induced breakouts.
OR
Maybe if I took fewer AP classes I would have fewer stress-induced breakouts.
Which one of these sentences is correct? The second one. Why? Because we said so.
Oh, that explanation doesn't satisfy you? We expected as much, so we'll go on. You use less when you're talking about things you can't really count. And unless you have a time-turner like Hermione Granger, we're guessing you can count the number of AP classes you're taking.
We'd also guess that your stress-induced breakouts are pretty memorable (and annoying), so you can probably count those as well. That's why fewer is the correct word choice there.