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The words a, an and the are usually called 'Articles'. They are called articles because they have some special significance on a sentence. Now, they are also called 'determiners'. They are used before a singular noun to determine that noun as singular. 

They are two types of articles:
(1) Definite article - the
(2) Indefinite article - a, an

Indefinite article: If we wish to generalize a noun, we use the Indefinite article.

Definite article: If we wish to particularize the noun, we use the Definite article.

To understand the two types of articles, lets first know the meaning of the word 'define'. It means - to explain the meaning of any issue, word, matter. The noun form of 'define' is 'definite'. Definite means  something that is already defined. And vice-versa 'indefinite' - .not clearly expressed or defined. As I mentioned, the is a definite article, so it is used after one particular object is already mentioned/defined. a / an is used before a singular item that is not particularly mentioned before; but, if a particular noun is mentioned two times or more, then, after the first time a / an is used, the will be used afterwards.

a  versus an
The choice between a and an is determined by sound. A is used before a word BEGINNING