A, an and the are Adjectives although in grammar they are called articles.

"The" is called a definite article.
When we say "Put the book on a table," we ask you to put a particular book on any table.
When we say "Put a book is on the table," we ask you to put any book on a particular table.
When we say "The book is on the table," we point out a particular book on a particular table.

"A" and "an" are called indefinite articles. When we say I have a book, no specific or particular book is indicated.

Choice of 'a" or "an"

Use an before all vowel sounds except long u and before words beginning with silent h.