What is a Noun?
The universe is made up of materialized nouns. Objects and beings, concepts and more, -- anything that exists in the physical world, and even in the imaginary worlds of the mind or in the virtual worlds of cyberspace, have a name. Every new thing, idea, or being that is encountered, on this world or at the edges of the known universe, or in the depths of an atom, gets named.
So, nouns are names. Like one's given name, they identify the object. It is nearly impossible to think of any name that attaches only to one entity; that is, the word ball is not used once, for one ball and then thrown away. Imagine what a mess it would be if we had to come up with a new word every time we encounter another ball... and then think of the disaster it would be if this were true for all nouns or names of things! Language is quite an economical tool, isn't it? Even though people are unique -- there are no two of anyone, not even identical twins who do share the exact same DNA -- the people's names, first, last, middle, or even nicknames, are not unique.
Nouns come in two basic types: proper nouns (a person's name) and common nouns. What common nouns name can be named too: persons, animals, places, things, feelings, phenomena and abstractions. There are nouns for animate beings and inanimate things. There are generic nouns and nouns that get specific. Consider the following examples.
First, the nouns referring to types of persons, distinct from their personal name, such as John or Mary: friend, family, police, stockbroker, farmer, musician, crook. Notice that some nouns refer to a group of persons. They are known as collective nouns. In American English, they tend to be all singular but refer to a group, such as family. In England, family is plural.
As for animals, consider: dog, tiger, sheep, fish, shrimp. Notice that the last three nouns are used for both singular and plural, although many people seem to be forgetting that! Other animals have different names entirely for when they are singular or plural: goose and geese, for instance. Some animal names distinguish male and female: cow, bull; rooster, hen. In English, only natural gender occurs, not grammatical gender, as in Spanish -- which also can have different names for male and female animals of the same species: vaca, toro; gallo, gallina.
Examples of places, as types of places and not the proper name of a place, such as New Delhi, which is a proper place name (toponymic): river, city, hill, lake, ocean, and so forth.
When naming things, the list is practically infinite, but you should remember that there are nouns that classify whole groups of like objects, just as there are that refer to specific types, just as in the world of living things. Things can include tools -- and even the word tool itself, tree (generic) or oak (specific).
Feelings include emotions and sensations: love, happiness, itch, ache.
Phenomena include natural and mental: lightning, flood; thought, dream.
Abstractions do not refer to physical objects at all. Love could be considered such an abstraction, but it is best classified as a feeling -- and it is connected with physical causes -- brain chemicals and the like! Examples of abstractions are often the vocabulary of philosophy and religion: truth, faith; temperance, fortitude, prudence and justice; virtue.
Nouns function either as doers -- grammatical subjects, or they are acted upon, making them objects.