'Fewer' - for countable nouns
'Less' - for uncountable nouns
However, these days people tend to use 'less' for both countable and uncountable.
Some people still consider this a mistake.
Many people use 'less' for both countable and uncountable nouns nowadays, but it looks good if you use 'fewer' correctly (see another real-life Task 1 summary example of 'fewer').
Write your own example so that it's memorable for you.
Examples