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.

A or An?

Use   a   before   words   that   begin   with consonant sounds:

• a tree   
• a year
• a hotel   
• a university
• a large elephant

Use  an before words that begin with vowel sounds:

• an apple   
• an hour
• an umbrella       
• an old man

DEFINITE ARTICLE (THE)

The indicates that the noun that follows has already been defined in some way. The introduces...

1. Specific, known nouns:

• the woman next to me
• the plants in her yard
• the day after tomorrow
• the beauty of the world

2. One-of-a-kind nouns:

• The sun is shining.
• I want to buy the biggest house in Paris.

3. Nouns  representing a  general  class of things:

• I play the piano.
• Every day, I go to the office.
• The computer is an amazing invention!

NO ARTICLE

Omit articles ...
1. Before nonspecific plural count nouns.

• There are apples on the tree. 
• Computers can do many things.

2. Before nonspecific noncount nouns.

• Honesty is very important to me.
• I love milk!

3. Before some expressions. "I went to school" means that I went to my school to study, whereas "i went to the school" means that I went to some particular school building.