As far as most people are concerned less and fewer are effectively the same word. You might say that less is, more or less, fewer.
Whoa there dobbin. That's categorically not so according to some language purists.
When to use the word less and when to use fewer is one of the pet hobby horses of those who believe the choice to be authorized by grammatical decree. Such folk are known as prescriptivists by those who agree with them, and as the word police, grammar pedants/Nazis and so on, by those who don't.
Prescriptivism is the view that the English language has a standard grammatical form which needs to be guarded and that deviations from it are to be deplored and rooted out. Things that annoy prescriptivists include split infinitives, greengrocer's apostrophes, prepositions at the end of sentences, verbing, and the cause we are interested in here, the proper use of less and fewer. They claim that their approach is guided by the need for clarity and meaning in what is said and written.
In the other camp are those who view language as fluid and continuously changing by everyday use and that the application of archaic and arbitrary rules drains it of life. This group believe that a creative and dynamic approach keeps the language vigorous and enjoyable, without any loss of clarity. They view prescriptivists as self-appointed pedants who, having taken the trouble to learn the rules, feel the urge to impose them on everyone else.
[Note that, although I attended an English grammar school, where the said rules were taught to us by the uncompromising method of hitting us with a stick when we got them wrong, I am firmly in the second camp.]
So, let's get back to less and fewer and let battle commence.
The basic rule that governs whether you use less or fewer is that 'fewer' should be used for things that are counted and 'less' for things that are measured.
Okay, so what's the difference? A countable noun is something that there can be many of, that is, something you can form and plural of, like children or shoes for example. So, applying the rule, we would say:
Fewer children are studying cookery these days.
I have fewer shoes than my wife.
Uncountable nouns are singular, for example music or milk. So we would say:
I listen to less music than my children.
My cows are giving less milk than they used to.
But, and with language there's always a but, fewer isn't used when talking about things that, although plural, are usually considered as a whole rather than a collection of individual items, for example, sums of money. So we would say:
Less than fifty pence; that's a bargain.
The above 'but' comes into play because, when talking about money, we are considering the total amount as a singular item. So, when referring to fifty pence we aren't usually meaning fifty individual pennies but a total amount of money. So, such items are singular, hence we use 'less'.
The same goes for distance, time and ages, hence:
It's less than two miles to the Peak District from here.
Children aged less than four don't go to school.
Also, fewer isn't used for numbers on their own:
My weight is under 80 kilos.
Three is less than four.
So, that's the rules out of the way.
So, why the rule? Let's step back to 18th century England, at which time, spelling, pronunciation and grammar were highly variable, with some regional dialects being unintelligible to outsiders. In the same way that the advent of railways in 19th century England required the imposition of a standard time, the publishing of books and newspapers in the 18th century induced writers to standardise the language.
Samuel Johnson published his Dictionary of the English Language in 1755. Jonathan Swift, deploring what he called 'the corruption of the English tongue', was one of several notables who proposed an academy to arbitrate over what was correct English. This was an attempt to follow Cardinal Richelieu's Académie Française, which rules on matters pertaining to the French language. The middle years of the 18th century saw a deluge of books on grammar, dictionaries and rule and style guides aimed at expunging 'barbarisms' and 'corruptions'.
At this point, step forward Robert Baker. Not much is known about Baker - he wasn't a renowned writer or grammarian and he held no especial status in society. And, at this point, I'd like to introduce a portrait of him but there doesn't appear to be one, which must say something about his status.
In the forward to a 1770 book Baker wrote a plea to King George III:
"My first proposal is that your Majesty would... establish in London an Academy of the Nature of that of the Belles Lettres at Paris... Our language, as has been often observed, is manly and expressive, but our writers abound with Incorrectness and Barbarisms..."
As it turns out, Baker was pleading for a cause that was already largely lost. It had been backed by Queen Anne earlier but, after she died in 1714, support began to peter out. Nonetheless, Baker persisted. In his Reflections on the English Language, 1770, he lists 127 rules, including this, Rule 47:
This word is most commonly used in speaking of a number; where I should think fewer would do better. No fewer than a hundred appears to me not only more elegant than no less than a hundred, but more strictly proper.
This opinion appears to be the source of all subsequent statements that there is a rule of grammar relating to less/fewer - there are certainly no known prior examples of it in print.
Well, we don't. Baker made the rule up for no obvious reason and, also for no clear reason, people accepted it. He claimed no expertise as a grammarian and only expressed it as a subjective preference.
We might wonder why we need such a rule - the substitution of 'fewer' for 'less' adds nothing to intelligibility.
When considering things that exceed other things we use 'more' and this word is considered sufficient for both countable and non-countable things. So, while the rule would have it that we should say 'fewer cows; less milk' grammarians are content with 'more cows; more milk'.
For centuries prior to Baker's book people had been happy to say 'less cows; less milk' and the sky didn't fall.
The English language needs rules in order for us to understand each other, but for no other reason. Baker's 'less versus fewer' is a rule for rule's sake and should be sent into the same linguistic oblivion as 'never split an infinitive' and 'never end a sentence with a preposition'. What became of his other 126 rules isn't clear.
Perhaps prescriptivists might start to lobby for a new 'extra cows; more milk' rule, which would after all, be as valid as Baker's Rule 47.