Both these words are adjectives, but "few" is usually used to describe countable nouns while "less" is used to describe noncountable nouns. Countable nouns are often physical whereas noncountable ones are often abstract or nonphysical:
The industrial trend is in the direction of more machines and fewer people. ("People" are countable.)
Less destruction was caused by the earthquake than one would have expected. ("Destruction" is noncountable.)
If you memorize the phrase "few units = less quantity," you will remember the distinction—"few" is for countables occurring in units, while "less" is for noncountables occurring in quantity.
As often happens in English, exceptions do arise. Sentences involving periods of time, sums of money, or specific measurements usually require "less":
The sonde was lowered less than 50 feet.
Excavation took less than two weeks.