The words a, an, and the are generally called articles and sometimes classed as a separate part of speech. In function, however, they can be grouped with the demonstrative adjectives that are used to point things out rather than describe them.
The is called the definite article because it points out a particular object or class.
This is the book I was talking about.
The dodo bird is extinct.
A is called the indefinite article because it points out an object, but not any particular specimen.
a book, a dog, a lawn mower
The indefinite article has two forms:
A is used before words beginning with a consonant sound or an aspirated h:
a car, a lamb, a hope, a habit, a hotel
An is used before words beginning with a vowel sound:
an ape, an image, an untruth, an honorable man