If you want the dictionary definition, just double click on any word.
Everyone gets this wrong - including native speakers. The general rule is to use fewer for things you can count (individually), and less for things you can only measure
There were fewer people in the class than usual. (People can be counted.)
There were fewer days below freezing last winter. (Days can be counted.)
I drink less coffee than she does. (Coffee cannot be counted individually it has to be measured).
It looks bad when there's less information about your company on a site than there is for other companies.
!Note - The lines between less and fewer are getting blurred, i.e. a sign in a shop might say, "10 items or less," because "10 items or fewer," seems pompous to many people. Or maybe it's because less takes fewer letters.
For your tests or exams, "less" answers the question, "How much?" "fewer" answers the question, "How many?"