The articles in English are words that combine with a noun to indicate the degree of definiteness (specificity) of the reference being made by the noun. The articles in English include the definite article the and the indefinite articles a and an (and sometimes some). The use of the definite article indicates that the speaker assumes the listener knows the identity of the noun, because it is obvious, because it is common knowledge, or because it was mentioned in the same sentence or an earlier sentence. The use of an indefinite article indicates that the speaker assumes the listener does not know the identity of the noun.
In English, nouns must in most cases be preceded by an article that specifies the presence or absence of definiteness of the noun. The definite article is the in all cases other than generic references, which use the zero article (i.e., the absence of an article), while indefiniteness is expressed with a or an for singular nouns or the zero article for plural or non-count nouns.