less vs fewer

Less- only used when you are talking about something that is not quantifiable (can not be counted or measured).

There is less water in your glass.

There is less snow on the ground now that the sun has come out.

Fewer- used for anything that is quatifiable (can be counted or measured)

She has fewer items in her cart.

We had fewer inches of snow this year than last year.

* More often than not you actually want to use “fewer” not “less”.