a, an, the

The Literal meaning of the word article is a little joint from the Latin 'artus' meaning a joint or limb.
 

There are three articles in English grammar, a, an, and, the.

1.'The' is known as the definite article as it shows that the noun which follows is a particular or special one of its kind. The definite article can be used for both singular and plural nouns

Example :

Where is the book?

Where are the boys?

2. 'A' , 'An' are known as indefinite article, as they indicate that the noun which follows could be any one of that type of thing. The indefinite article may only be used with the singular noun.

Example :

 Alice works in a bank.    (not 'in bank')

 Can I ask a question?    (not 'ask question')

Note : Use An (not 'a') before a / e / i / o / u:

Do you want an apple or a banana?

I'm going to buy a hat and an umbrella.

Some Exceptions : 

An hour, A university, A European ,.....