In linguistics, a count noun (also countable noun) is a noun that can be modified by a quantity and that occurs in both singular and plural forms, and that can co-occur with quantificational determiners like every, each, several, etc. A mass noun has none of these properties: It cannot be modified by a number, cannot occur in plural, and cannot co-occur with quantificational determiners.
Below are examples of all the properties of count nouns holding for the count noun chair, but not for the mass noun furniture.
Occurrence in plural.
There is a chair in the room. (correct)
There are chairs in the room. (correct)
There is chair in the room. (incorrect)
There is a furniture in the room. (incorrect)
There are furnitures in the room. (incorrect)
There is furniture in the room. (correct)
Co-occurrence with count determiners
Every chair is man-made. (correct)
There are several chairs in the room. (correct)
Every furniture is man-made. (incorrect)
There are several furnitures in the room. (incorrect)
Some determiners can be used with both mass and count nouns, including "some", "a lot (of)", "no". Others cannot: "few" and "many" are used with count items, "little" and "much" with mass nouns. On the other hand, "fewer" is reserved for count and "less" for mass (see Fewer vs. less), but "more" is the proper comparative for both "many" and "much".
The concept of a "mass noun" is a grammatical concept and is not based on the innate nature of the object to which that noun refers. For example, "seven chairs" and "some furniture" could refer to exactly the same objects, with "seven chairs" referring to them as a collection of individual objects but with "some furniture" referring to them as a single undifferentiated unit. However, some abstract phenomena like "fun" and "hope" have properties which make it difficult to refer to them with a count noun.
Classifiers are sometimes used as count nouns preceding mass nouns, in order to redirect the speaker's focus away from the mass nature.