Traditionally in English, an article is usually considered to be a type of
adjective.
In some languages, articles are a special part of speech, which cannot
easily be combined with other parts of speech. It is also possible for
articles to be part of another part of speech category such as a
determiner, an English part of speech category that combines articles and demonstratives (such as 'this' and 'that').
In languages that employ articles, every
common noun, with some exceptions, is expressed with a certain
definiteness (e.g., definite or indefinite), just as many languages express every noun with a certain
grammatical number (e.g., singular or plural). Every noun
must be accompanied by the article, if any, corresponding to its definiteness, and the lack of an article (considered a
zero article)
itself specifies a certain definiteness. This is in contrast to other
adjectives and determiners, which are typically optional. This
obligatory nature of articles makes them among the most common words in
many languages—in English, for example, the most frequent word is
the.
[1]
Articles are usually characterized as either
definite or
indefinite.
[2]
A few languages with well-developed systems of articles may distinguish
additional subtypes. Within each type, languages may have various forms
of each article, according to grammatical attributes such as
gender,
number, or
case, or according to adjacent sounds.
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http://en.wikipedia.org/wiki/Article_%28grammar%29
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