Fewer is for things that can be counted in numbers. Less is for non-numbered things. It's so easy that it really disturbs me when people get it wrong.
Fewer people. Fewer zoos. Fewer cigarettes. Less hatred. Less abuse. Less misuse of power.
Fewer people. Fewer zoos. Fewer cigarettes. Less hatred. Less abuse. Less misuse of power.
Flip flopping it around. The last three are now countable. The first three, while maybe verging on nonsensical, are grammatically correct. Less zoo, for example, might be used in the context of closing down part of a zoo. 'I have less cigarette left than Joe', also makes sense, if we are comparing cigarette lengths.
On another note, see how "fewer times" and "less time" means different things (might help clarify the distinction to some.)
(Non native speaker here though.)