A noun is the name of a person, place, thing, or idea.
man... college... house... happiness

The first real word you ever used probably was a noun-a word like mama, daddy, car, or cookie. Most children begin building their vocabularies with nouns. A noun names something: a person, place, or thing. Most other parts of our language either describe nouns, tell what a noun is doing, or take the place of a noun.

Nouns have these characteristics:

They are abstract or concrete.
They are proper or common.
Most are singular or plural, but...
Some are collective.

In English, nouns are often preceded by noun markers--the articles/adjectives a, an, the, or some for example; or possessive words like my or your. A noun always follows a noun marker, though adjectives or other words may come between them:

my former roommate
a sunny June day
an objective and very thorough evaluation
some existential angst

Because of their noun markers, you could safely guess that roommate, day, evaluation and angst are nouns (even if you didn't know what angst was). Although not all nouns are preceded by markers, you can use a noun marker test to identify many, including abstract nouns. Consider this example:

Enthusiasm and willingness to work hard are a remedy for the existential angst of many students.

Remedy is marked as a noun by the noun marker a. Angst is preceded by the noun marker the. Students is preceded by the adjective (adjectives modify nouns) many. Test the remaining words: can you have an enthusiasm or some enthusiasm? Certainly: "I have an enthusiasm for snowboarding" or "Show some enthusiasm!" So enthusiasm is a noun. Can you have an and or some and? Uh, no. So and is not a noun. Can you have a willingness or some willingness? Sure, you can have "a willingness to learn"; willingness is also a noun.

Every noun is either abstract or concrete.
Nouns like enthusiasm, willingness and angst are abstract nouns. Abstract nouns name things we cannot see, touch, or detect readily through our senses. Abstract nouns name ideas (existentialism, democracy), measurements (weight, percent), emotions (love, angst), or qualities (responsibility). Concrete nouns, on the other hand, name persons, including animals (cousins, Roger Rabbit), places (beach, Chico), or things we can see, touch, or otherwise detect through our senses (smoke, beer).

Every noun is either proper or common.
A proper noun identifies a particular person, animal, place, thing, or idea--Roger Rabbit, for example. The first letter of each word of a proper noun is capitalized. A common noun does not name a particular person or thing; rather, it refers to a whole class or type. Common nouns do not require capitalization.

Proper noun (capitalized)
Sierra Nevada Crystal Wheat is his favorite beer (common noun).
The Rooks and the Rangers are our local soccer and baseball teams (common noun).
Bidwell Park is one of the largest municipal parks (common noun).
Lundberg Family Farm is a sustainable, organic farm (common noun).

Most nouns are either singular or plural...
Most nouns are made plural with the addition of s or es. Thus, instructor becomes instructors, and class becomes classes. Some nouns have irregular plural forms: man becomes men, and woman becomes women. Child becomes children, and person becomes people.

Many people, both men and women, believe that having children will be a remedy for their existential angst.

Some nouns have the same form in both singular and plural: "A moose is crossing the river. No, wait--three moose are crossing the river!"

...but some nouns are collective.
A collective noun names a collection or group of things. Although a collective noun refers to a group of many things, it is usually singular in form. We think of a collective noun as singular because its members act in one accord:

The army is withdrawing from those Asian countries that are in negotiations.

Here, army is a collective noun referring to a group of many people acting with one will. We treat it as a singular noun. Countries is a plural noun. If several countries joined together to form an alliance, we could say this:

The Asian alliance is united in its determination to repel foreign invaders.

In some instances a collective noun describes a group that is not acting with one will, whose members rather are taking independent, divergent actions. In this case, the collective noun is treated as a plural to reflect the plurality of the members' actions:

The jury were unable to come to any consensus.

If the jury had reached a unanimous decision, we would have said:

The jury was unanimous in its verdict.