A and an are indefinite articles used to refer to a singular countable noun.

           An indefinite article means that we don't know which one, or it is
           not important.

           A is used before a consonant (b,c,d,f,g,etc.):          a  car
           An is used before a vowel (a,e,i,o,u) :                   an animal

                      *Tom lives in a town.
                       *Julie is reading an English book.
                       *We eat meat with a knife and a fork.

the

          The is a definite article used to talk about something specific.
           *The town where Tom lives is very big.
           *What book is Julie reading? She is reading the book Tom gave her.

          The is also used to refer to:
          *Rivers, seas, oceans: 
            the Mississippi, the Mediterranean, the Atlantic.
          *Nationalities :
            the British, the Americans, the Japanese