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 iPad.
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 1999, the Beatles or the IEEE (Institute of Electrical and Electronics Engineers).
Articles are also considered adjectives, because they add information about nouns.