As a simple definition, a noun is any word that names a person, place, or thing. This definition is somewhat vague, and many people also include that nouns can signify an event or idea. A noun is one of the major parts of speech, along with verbs, adjectives, adverbs, prepositions, and articles.
There are many different types of nouns. These include proper nouns, which refer to a specific person, place, or thing, and are indicated in English by a capital first letter, such as Nancy or Los Angeles; concrete nouns, which are nouns that describe a tangible object or person, such as the dog; abstract nouns, which describe anything that cannot be perceived using the five senses, such as love; countable nouns, which are nouns that may be pluralized, such as chairs; non-countable nouns, which are nouns that have no plural, such as bedding; and collective nouns, which describe a group of another noun, such as a murder of crows.
A noun usually takes one of two roles in a sentence, either that of the subject or an object of the sentence. The subject of the sentence can be looked at as the 'doer' of the sentence, the noun which acts in some way. The object of the sentence, which is an optional component, is a noun which is acted upon by the subject. An example might help to demonstrate these roles. In the sentence: The man rode the bike. the words man and bike are both nouns, with man serving the role of subject, and bike acting as an object.
People first learning about the parts of speech often find it difficult to differentiate between nouns and adjectives such as yellow or outside. The trick to telling the difference is by identifying the role the word is playing in the sentence, especially since in English the same word may often be used as a different part of speech depending on context. For example, in the sentence: The yellow man rode the bike. the word yellow is modifying the noun man, and so is being used as an adjective. In this context, yellow does not name a person, place, or thing -- rather, it describes the state of the man.
On the other hand, in the sentence: The man liked yellow. both the words man and yellow are being used as nouns, with man the subject and yellow the object. In this context, the word yellow is describing a thing -- namely, the color yellow.
In English, nouns are rather simple. They tend to come in only two forms, singular and plural. A regular noun can be made plural simply by adding an s or es to the end, such as in the combination hat/hats, while irregular nouns follow different rules which must be learned individually for each noun, such as the combination goose/geese.
In many languages, in addition to being plural or singular, a noun may also have a specific gender or class associated with it, which affects how it might be declined or interact with other parts of speech. In English we retain the idea of gender, but use it only in rare cases, such as steward/stewardess or waiter/waitress, though it is becoming more and more common to use gender-neutral forms of these words as well.
A noun may also have different cases, which help inform the role the noun plays in a sentence. In Latin, for example, the case of a noun shifts depending on whether it is the subject or object of the sentence. In English we rely more on the placement of the word and supporting words such as prepositions to help relay this information. English nouns do, however, still make use of a possessive or genitive case, which is indicated by adding 's to the end of a noun. Nouns make up the basic building blocks of sentences, and in most cases, for a sentence to be grammatical it must include at least one noun subject and one verb.