Actually, they are two different articles: definite (the) and indefinite (a, an).

Indefinite articles define a noun as unspecific, definite as specific.

1. Indefinite articles:

• article 'a'
  is used before a singular noun which starts with a consonant sound.
  example - a car ( It can be any car. It is not specified which car we are talking about).
• article 'an'
  is used before a singular noun which starts with a vowel sound (a,e,i,o,u).
  example - an apple ( It can be any apple: big, green, red, juicy, rotten, ripe... we don't know)

1. Definite article - the
   The definite article can be used with singular, plural, or uncountable nouns.
   example - The movie we watched last night was interesting. (not any movie)

Hope that the comment helped.