An and a are indefinite articles and The is definite article.
<em>Indefinite articles</em> (an and a) are used for subjects which aren't specific. For example: a ball, a book, a table, a watch, a star, a plane, a man...
an airplane, an elephant, an apple, an egg, an orange...
<em>Definite article (The)</em> is used when we are describing something specific. For example: The air, the weather, the watch (When you know about what specific watch are you talking about), the book (specific book), the boys, the man...
The word AND is used as a conjuction, not an article.
