We divide them into "indefinite" and "definite" like this:
indefinite articles definite article
a an the
We use "indefinite" to mean not sure, not certain. "Indefinite" is general.
We use "definite" to mean sure, certain. "Definite" is particular.
When we are talking about one thing in general, we use a or an. When we are talking about one thing in particular, we use the