Articles in the English language are the definite article the and the indefinite articles a and an. 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 be told the identity of the referent. In some noun phrases, no article is used.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/article