Few things distinguish the educated person from the rest of humanity like failure to understand the difference between mass nouns and count nouns. And yet, the difference is so simple.
Beloved of Count Von Count, Count Nouns (no, Virginia, they were not named after him) are nouns denoting discrete items. Why does the Count love them so? Because they can be COUNTED.
How many sheep, we say. How many cookies? How many cats/ racing car drivers/ scandals involving Federal politicians? A count noun is a noun of which such a question can be asked, and which can be enumerated. Ten thousand head of sheep. A dozen cookies. Four cats, three racing car drivers... you get the picture? However many of something there are, things of this kind can be counted. Even if it takes a long, long time.
Mass Noun, What
Not all nouns denote things that can be counted. Consider, for example, water. Or milk, or pinot grigio. You can't really say 'how many water'. Not if you're an English speaker, that is; it may well be possible in some languages, but it is English with which we are concerned today. You might, in colloquial usage, say 'how many pinot grigio', but a moment's thought will reveal that this is actually a kind of shorthand for 'how many bottles/glasses/barrels of pinot grigio'.
How do we refer to quantity with these non-countable nouns? We use measures of quantity rather than number. How much water? we ask. The answer will differ in form, too. Instead of a simple number, we must answer with a number and a unit. For example, ten gallons. Forty bottles. Nouns of this kind are known as Mass Nouns.
The Difference At A GlanceIt's easier than anything to tell which kind of noun is which. All you need to ask yourself is which question you would ask in relation to it - how much (mass noun) or how many (count noun)?
But Wait - There's More!
There is indeed more - and the other thing, too. Let's consider 'more' first, for it is very simple. Whether we are counting (bubbles) or measuring (champagne), the comparative 'more' is the same. One glass has more bubbles than the other. One glass has more champagne than the other. No problems there, right? Right.
The Other Side of the Coin
Where we start to run into problems is when we use a comparative that describes a smaller quantity or amount, for here the same word cannot be used in all cases.
Mass nouns are the only ones that correctly take the comparative 'less'. You can have more or less water, more or less oxygen, more or less intelligence.
Count nouns, on the other hand, take the comparative 'fewer'. More people, fewer people. There are fewer pencils on my desk than there were yesterday.
One Word More
This seems a good place to note that 'more' and 'less' are also the comparatives used with adjectives, where those adjectives do not form their own comparatives. In this way, adjectives are a kind of cousin to the mass noun.