Fewer is the comparative form of few, and is used when denoting people or countable things.
There are fewer books here today.
Less, on the other hand, is used with numbers, with expressions of measurement, or with mass nouns referring to things that can not be counted. Using less to refer to things that can be counted is incorrect in standard English.
I have less than 500 songs on my ipod.
The store is less than four miles away.
Less music is played when I am away.
Incorrect: Less people showed up than I had predicted.
