An

'An' is same as 'a' in its meaning, but with different form and usage. In English Grammar, many grammarians place these words as adjectives, but many categorize them as articles.

Both the words are used as indefinite articles with only one difference that 'a' is used before words beginning with a consonant but 'an' is used before words beginning with vowel sound. Other rules of using an indefinite article may apply to both 'a' and 'an'.

'An' is also used as suffix and prefix, eg anemia, Indian etc.