Indefinite Articles
              'a or an' are indefinite articles and are used when to say about not a definite person, place or thing.

Example : I saw a girl.(any girl --not definite)
              He gave
me an orange.

Definite Articles
              'the' is called definite article and is used when to say about a definite person, place or thing.

Example : I saw the girl who secured the first rank.(girl --is definite)

Usage Of Definite Articles