Most of us learned the classic definition of noun back in elementary school, where we were told simply that “a noun is the name of a person, place, or thing.” That's not a bad beginning; it even clues us in to the origin of the word, since noun is derived ultimately from the Latin word nōmen, which means ‘name’.
As we see from its dictionary definition, a noun can name not only a physical thing but also abstract things such as a state (happiness) or a quality (beauty). The word is defined further in terms of the way it functions in the language—as a subject or object in a sentence or as the object of a preposition. In any of those positions, it can be modified by an adjective or adjective phrase: a talented but quirky artist.
Nouns are typically said to fall into two categories: proper noun and common noun. A proper noun designates a particular person, place, or thing and is normally capitalized: Shakespeare, Mexico, the Pentagon. A common noun refers to a generic person, place, or thing: teacher, classroom, smartphone. The plural form of a common noun names a set or group. (Proper nouns are pluralized only in special circumstances: There are many Springfields in the United States. Oh, no, the Smiths are coming to dinner again.)
To form the plural, most common nouns simply add an -s (teachers, classrooms, smartphones). Some nouns ending in –o (but not all) add -es. Nouns ending in the sounds or also have plurals ending in -es (bus/buses, ash, ashes, judge/judges). Several nouns form the plural in a different way. These include child/children, knife/knives, and a number of others. Some nouns have a plural form identical to that of the singular: sheep/sheep. Seven English nouns form their plural by changing the vowel in the middle of the word: woman/women, man/men, goose/geese, tooth/teeth, foot/feet, louse/lice. (Can you think of the seventh one?*) And then, of course, there are nouns borrowed from other languages that keep their non-English plurals (bacterium/bacteria, chapeau/chapeaux, kibbutz/kibbutzim).
But not all nouns can be pluralized. Thus we have another way to categorize nouns. Those that can be thought of in the plural are called count nouns; the things they name can be counted and enumerated. Other nouns, called mass nouns or noncount nouns, name things that are usually not counted, even when the amount grows larger. This class includes nouns that refer to a substance (water, sand, oxygen, electricity), a quality (kindness, honesty), or an abstract concept (happiness, health). There are exceptions: some substances can be spoken of in the plural if you are referring to various kinds (The wines of France are known throughout the world) or to units or containers of the substance (We’ll have three coffees and two teas).
Certain other nouns that name something relatively concrete, like furniture, flatware, hardware, and software, are also treated as mass nouns. This means that in English we do not say “This computer comes with the latest softwares.” Nor do we say “I’m buying a furniture” (although we can buy a couch or a table), since mass nouns normally cannot be immediately preceded by “a,” “an,” or a numeral. Instead, we use the singular form even when referring to large quantities, saying things like “a lot of software” or “too much furniture.” This distinction between count nouns and mass nouns, complex though it may seem, is pretty much absorbed automatically if you grow up speaking English. But it can be one of the most difficult things to assimilate for people learning English as a foreign language. The answer? Read, read, read. And listen.