An article is a little adjective which individualizes the noun, a boy, an apple, the crowd.
A which is used before consonantal sounds and an which is used before vowel sounds are called indefinite articles because they
individualize without specializing. The is called the definite article because it both individualizes and specializes.
A may be used before o and u if the sound is really consonantal as
in such a one, a use, a utility. An may be used before h if
the h is not sounded, for example, an hour but a horror.