The words a, an and the are types of adjectives called articles.
A and an are called indefinite articles.
A is used before singular count nouns.
A car, a book, a child, a holiday
An is used with count nouns beginning with a vowel sound.
An apple, an elephant and notice; an hour but a European. ( the 'h' in hour is silent. The 'e' in European is not a vowel sound)