There are three articles in the English language: the, a, and an. These are divided into two types of articles: definite (the) and indefinite (a, an). The definite article indicates a level of specificity that the indefinite does not. “An apple” could refer to any apple; however “the apple” is referring back to a specific apple.

Thus, when using the definite article, the speaker assumes the listener knows the identity of the noun’s referent (because it is obvious, because it is common knowledge, or because it was mentioned in the same sentence or an earlier sentence). Use of an indefinite article implies that the speaker assumes the listener does not have to be told the identity of the referent.

There are also cases where no article is required:

• with generic nouns (plural or uncountable): cars have accelerators, happiness is contagious, referring to cars in general and happiness in general (compare the happiness I felt yesterday, specifying particular happiness);
• with many proper names: Sabrina, France, London, etc.

Watch this quick introduction to indefinite and definite articles and the difference between the two: