This one is pretty straightforward, but I hear people getting these two words confused. Time for a quick review.
Fewer is an adjective. Period.
Less can be either an adjective or an adverb. I've never heard anyone mistake fewer for an adverb, so this is not where the confusion lies.
The question is which of the two adjectives to use when modifying a noun. Is it "fewer problems" or "less problems"? They both sound like they could potentially be correct. At least, neither adjective seems jarringly wrong.
Here's the rule:
Use "fewer" with countable nouns and "less" with non-count nouns.
What is a non-count noun? Anything which wouldn't normally be counted in individual or finite items. Water is an example of a (usually) non-count noun. You would ask for some water and not three water. This means that you would use "less" to modify water. For example, the tub holds less water (not fewer water). The same holds true for gas.
On the other hand, you don't want to use "less" to modify a countable noun. I heard an advertisement the other day state that something had "less calories". They really meant to say "fewer calories". It's not that they were incomprehensible. I understood what they meant, but they didn't exactly fill me with confidence.