There are three articles: the adjectives a, an and the. The is called the definite article as it is used to indicate something specific; whereas, a and an are called the indefinite articles as they are used to indicate something unspecific.
I fell over the chair again. (a specific chair, i.e., one known to the reader)
Can you pass me a chair. (an unspecific chair - i.e., any chair)
Use an (as opposed to a) only when the next word starts with a vowel sound.
It was an unicorn.
(Unicorn starts with a vowel but not a vowel sound.)
Glossary of grammatical terms