An article is a word used before nouns to specify them as definite or indefinite. There are three articles in English: the, a/an, and no article. The choice of article depends on whether the noun is countable/uncountable and singular/plural. The definite article "the" is used for specific or unique nouns, while the indefinite article "a/an" is used for nonspecific nouns. No article is used for uncountable or plural nouns. Proper use of articles is important for clear communication.