Less= (1) used to form the comparative of some adjectives and adverbs: ‘Less expensive’ ‘Less interesting’ (2) a quantifier meaning not as great in amount or degree (comparative of ‘little’ usually used with mass nouns): ‘He had less time to spend with his family.’ (3) with subtraction of; minus: ‘Ten less three is seven.’ (4) lower in importance or rank or quality (usually preceded by ‘no’): ‘This dress is no less than perfect.’
Use less with an uncountable noun and fewer with the plural form of a countable noun: ‘Less traffic would mean fewer accidents.’
Less is widely used in place of fewer in everyday conversation, but careful users regard this usage as non-standard. (It is nonstandard in some uses but often idiomatic with measure phrases): ‘Less than one week.’