An article is a word used before a noun that indicates whether or not the reference is to a specific entity or entities or an unspecific one. 

There are only three articles, a, an and the. A and an, the indefinite articles, are used to refer to a single and unspecified entity. A is used with nouns that start with a consonant sound, as in a battery, while an is used with words that start with a vowel sound, as in an iPhone.

The, the definite article, refers to a specific entity or entities. The single entity may also be made up of multiple individual entities, as in, for example, the class of 1987, the Beatles.