Grammarians are not always agreed as to the grammatical status of the article in Modern English.
In structural grammars the article is often dispensed with as a separate part of speech and absorbed into the adjective class.

The name "determiners" is then given to closed system items, which, functioning as adjuncts, show their head-words to be nouns. The most central type of "determiner" is that to which we traditionally give the name article.

Article introduces a noun and indicates how specific the noun is.
English has two articles: the definite article "the" and the indefinite article "a" (or "an").

INDEFINITE ARTICLE (A, AN)

Use a (or an) only before singular count nouns.

A (or an) means, approximately, "one." "I saw a cat" and "i saw one cat" describe the same event, but "I saw one cat" emphasizes that there was only one cat, not two.

Use the indefinite article to introduce...

1. Nonspecific, unknown nouns:

• He entered a brown building. 
• There is an apple on the tree.

2. One of a general group:

• A computer can do many things.