In grammar, an article is a short word used to show that we know the thing or things being referred to (the definite article) or that we don't know the thing being referred to (indefinite article). They are also used to talk about particular things or things in general. Articles belong to a group of words called determiners.
Unlike some other Western European languages, there is only one definite article in English: the. It can be used to refer to singular (the book) or plural (the books). It is also the commonest written word in the English language.
One thing to bear in mind, however, is the difference in pronunciation between the used before a vowel sound (/ði:/), and before a consonant sound (/ðə/). The importance here is the sound that the following word begins with, not the spelling. Thus, the agency /ði: eidʒənsi/; the hour /ði: aʊə/; the oven /ði: ʌvən/, but the book /ðə bʊk/; the unit /ðə ju:nit/; the one thing /ðə wʌn θiŋ/.
It should also be pointed out that if we want to stress the following word, we often use /ði:/, even if that following word begins with a consonant: It's the best place in town! (/ði: best/).
There are two indefinite articles in English: a and an. They are both singular in meaning. The words some and any can be used as the plural forms of a/an.