What are Nouns?
The simple definition is: a person, place or thing. Here are some examples:

* person: man, woman, teacher, John, Mary
* place: home, office, town, countryside, America
* thing: table, car, banana, money, music, love, dog, monkey

The problem with this definition is that it does not explain why "love" is a noun but can also be a verb.

Another (more complicated) way of recognizing a noun is by its:

1. Ending
2. Position
3. Function

1. Noun Ending

There are certain word endings that show that a word is a noun, for example:

* -ity > nationality
* -ment > appointment
* -ness > happiness
* -ation > relation
* -hood > childhood

But this is not true for the word endings of all nouns. For example, the noun "spoonful" ends in -ful, but the adjective "careful" also ends in -ful.

2. Position in Sentence

We can often recognise a noun by its position in the sentence.

Nouns often come after a determiner (a determiner is a word like a, an, the, this, my, such):

* a relief
* an afternoon
* the doctor
* this word
* my house
* such stupidity

Nouns often come after one or more adjectives:

* a great relief
* a peaceful afternoon
* the tall, Indian doctor
* this difficult word
* my brown and white house
* such crass stupidity

3. Function in a Sentence

Nouns have certain functions (jobs) in a sentence, for example:

* subject of verb: Doctors work hard.
* object of verb: He likes coffee.
* subject and object of verb: Teachers teach students.

But the subject or object of a sentence is not always a noun. It could be a pronoun or a phrase. In the sentence "My doctor works hard", the noun is "doctor" but the subject is "My doctor".

Sources:
http://www.englishclub.com/