The words a, an and the are used similarly to adjectives in that they modify nouns.  Articles are generally categorized as determiners, so I've never really considered articles to be adjectives. According to Wikipedia, articles used to be considered adjectives though.  
http://en.wikipedia.org/wiki/Adjective

Here is another website with relevant information:
http://owl.english.purdue.edu/owl/resource/540/01/

The website above says "Basically, an article is an adjective."   I agree with that, but as I see it, the word "basically" is significant. It qualifies the statement and changes the meaning just enough that it means something akin to "Articles are almost adjectives." 


Maybe someone else will have more input for you.