Indefinite article (a-an)

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This article is used to designate indefinite nouns from which we don't have reference.

A house  / A car  / A mountain

The indefinite article a-an is used to name singular nouns,  not plural nouns.
Use an when nouns begin with a vowel (a, e, i, o, u):

an animal / an eagle / an orange / an institute / an uncle

We use a when nouns begin with a consonant:

a table / a couch / a computer / a telephone

Note:
Sometimes, especially in the case of the letters h and u, it is the sound what defines whether the article is a or an:
An hour. In this case the h is silent and the next letter is a vowel.
A university / A uniform. In this case the u is pronounced as semiconsonant [iu].