Definiteness

DEFINITENESS is a grammatical category associated with nouns. By using DEFINITENESS, the speaker tells the listener whether the noun phrase is something general, or something specific that the listener should already know about.

(Please note: By tradition, all grammatical categories are written in CAPITAL letters.)

Articles are famous for being difficult to master, so don't feel bad if you are not very good at it.

Characteristics
Many languages do not have the grammatical category of DEFINITENESS, however in English it is very common.

3 types of DEFINITENESS exist:

Example 1: Definite
The speaker uses 'the' when the listener knows exactly what noun is being spoken about. If you are talking about the biggest, fastest, smallest, then you use 'the' because there can only be one biggest , there is only one fastest.

In this conversation, Susan talks about " an apple" because John can choose between 2 apples.

John answered "the red apple" because there is only one red apple. So John and Susan know which apple he is talking about. (At this time, John's choice in apple is defined, which is the root of the word DEFINITIVENESS.)

Example 2: Indefinite
In this conversation, Susan talks about " an apple" because John can choose between 4 apples.

John answered "a red apple" because there are 2 red apples. So John and Susan still don't know which apple he is talking about. (At this time, John's choice in apple is not defined, or "indefinite".)

Example 3: Generic
Generic DEFINITIVENESS looks very irregular, as you can see from the following examples:

Example 3.1.

 *  (No article)  Children today have no patience.
 * The youth of today have no patience.

Example 3.2.
I'm sorry, I do not know of any general rules to understand the generic case. :(
 * The blue whale is the largest mammal.
 * Blue whales are the largest mammals.
 *  (No article)  Women are better at seeing colors than men.

Maybe, if you want to say the noun class in singular you can start with 'the' (e.g. the blue whale, the youth of today); and If you want to say the noun class in plural, you do not say the article (e.g. Children, Blue whales, Women).

Other Ways of Showing DEFINITIVENESS

 * The speaker can use 'this' or 'that' instead of 'the' to refer to something definite.
 * The speaker can use 'some' or 'any' instead of 'a' to refer to something indefinite.

Also See
You can also read the page on articles for more information.