Modern English (sometimes New English or ne (ME)

Definite articles
In English, the only definite article is the word the. A definite article has several roles that it performs.
1. A definite article is used to refer to unique things or groups. A definite article can indicate that something is unique and only one of it exists.

• 
  She gazed at the moon. (The Earth only has one moon)
  
• 
  We sailed on the Pacific Ocean. (There is only one body of water named Pacific Ocean.)
  

2. A definite article is used to refer to specific things that both the speaker and the listener know the identity of.

• 
  Harold went to the store. (Both the speaker and the listener know specifically which store Harold went to.)
  
• 
  The kids are playing outside. (Both the speaker and listener know the identity of the kids.)
  

In writing and speech, the word the is typically used after the speaker or writer first introduces a thing that they are talking about. The word the then references back to this same thing. For example,

• 
  We saw birds by the lake. The birds were building nests. (The word the indicates that the birds in the second sentence are the same birds mentioned in the first sentence.)
  

 Indefinite articles
In English, there are two indefinite articles: a and an. A is used before consonants or consonant sounds, and an is used before vowels or vowel sounds. Indefinite articles are basically the reverse of definite articles and have several different roles.
1. Indefinite articles are used to refer to things that aren’t unique.

• 
  Raquel is a doctor. (Raquel is just one of the many doctors in the world.)
  
• 
  France is a country. (France is one of many countries.)
  

2. Indefinite articles are used to refer to nonspecific things.

• 
  I am looking for a job. (I am not looking for a specific job.)
  
• 
  Please hand me an envelope. (You can hand me any envelope you can find.)
  

In this same sense, indefinite articles are often used to refer to something as an idea or concept.

• 
  Going camping is a fun experience. (This sentence says that the act of “going camping” is an example of something that can be described as “fun experience.”)