The articles in English are the definite article the and the indefinite articles a and an (and sometimes some). Use of the definite article implies that 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 know the identity of the referent. In some noun phrases no article is used.
Articles are a special case of determiners in English; for information about this class as a whole, see English determiners.