The three articles in English are ‘a,' ‘an' and ‘the.' There three articles can be divided into definite and indefinite articles. ‘A' and ‘an' are indefinite articles and ‘the' is called the definite article.  ‘A' and ‘an' are indefinite articles. A and an signal that the noun modified is indefinite, referring to any member of a group. Indefinite articles are used with singular, countable nouns, in general. For example, A man is a mammal, A bird was sitting on the branch, I ate a banana for lunch, etc. ‘An' is used when you refer to nouns that begin with vowels (a, e, I, o, u). But pronunciation changes this rule. For instance we say ‘a unicorn' and not ‘an unicorn.' This is because the word is pronounced a younicorn. A is used when you refer to a nouns that begin with a consonant (there are 21 consonants). But once again pronunciation changes this rule. For instance we say ‘an hour' and not ‘a hour.' This is because the ‘h' in the word ‘hour' is silent. ‘The' is called the definite article. You use the when you know that the listener knows or can work out what particular person/thing you are talking about. The signals that the noun is definite, that it refers to a particular member of a group. For example - the apple that you ate was rotten; the apple is on the red mat, etc.