Though both of these refer to some kind of reduction, they are far too often misused. Less should be used in general broad terms, like less soil or land. Fewer is used with specific items, like fewer leaves or trees. Generally, if it can be counted individually, then fewer is correct. If it is a mass amount, then less is correct.

Less = bulk, mass, amounts, or quantity – or plural terms

Fewer = countable things or individual items – or singular terms

So, if your Mother gets fewer than 6 roses on her birthday, she might be OK with that. If she gets less roses than she did last year, she might worry about your finances.

I may have had fewer than twenty minutes to eat my lunch today, but I had less time for lunch yesterday.