There is no doubt that articles are adjectives since they modify the nouns after them. But articles have some special significance as determiners. Articles determine the standard of nouns.
There are two types of articles:
- Definite Article - the
The makes the noun something particular and definite.
- Give me the ball.
(Here, the speaker is telling someone to give him/her a particular ball about which the speaker and the listener both are aware. There is no possibility of the ball to be anything else rather than the speaker and the listener idea of that particular ball.)
- Indefinite Article - a & an
A & an - make the noun something general and indefinite.
- Give me a ball.
(Here, the speaker is telling someone to give him/her a random ball about which the listener is not particularly aware of, and s/he might ask ‘which/what kind of ball you want?’.)
- Give me an egg. (It can be any kind of egg – the possibility is open.)
Source: Learn Grammar