含义不同、用法不同。more的意思为更多的、更大的，可作副词，后面可接可数或不可数名词；less意为更少的，后接不可数名词，也可以用来修饰动词、形容词和副词；fewer意为较少的、几乎没，表示否定的意思，后接可数名词。
He'd like more time to think things over.
他希望有更多的时间把情况考虑周详。
Now she won't have to go out to work any more.
现在她不必再出去工作了。
How much will it cost, more or less?
这个大概得花多少钱？
It is less of a problem than I'd expected.
问题不像我预料的那么大。
Fewer people write with their left hand than with their right.
用左手写字的人比用右手的人少。
There seem to be fewer tourists around this year.
今年来访的旅游者似乎少了。