Indefinite article (a-an)

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

A house  / A car  / A mountain

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

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].