Less and fewer have essentially the same meaning, but they are used differently according to what they modify. Less is used with mass nouns, while fewer is used with count nouns. Look at the following example:
Example (less-correct usage): There is less milk than juice in the refrigerator.
In the example sentence above, milk does not consist of an amount that can actually be counted, so less is the appropriate word to use. This type of sentence rarely causes any problems for English speakers. However, in a sentence such as, “There are fewer people in class today than there were yesterday,” a great deal of confusion arises over what is correct. Many people would be tempted to say, “There are less people in class today than there were yesterday.” But remember that if the objects can be counted, always use fewer, not less. People in a class can be counted, so fewer is the correct choice for this sentence.