What is the difference between using 'a' or 'an'?

The indefinite articles [a type of
adjective] a and
an indicate a noun that is one of
many. Some grammar books refer to them as "noun markers." That is, a common noun that is
not specified as a certain one is preceded by
a or
an. When, for example, a person asks
for "a pencil," it does not matter which pencil the person has, just as long as it is a
pencil that works, probably (one of many).  However, if a person asks for "the pencil,"
he or she refers to a specific pencil, not just any one.  The article
the, therefore, is termed a definite
article.

In determining the use of
an as opposed to
a, whenever the noun that the article
modifies begins with a vowel, or a vowel sound--e.g. an original idea, an
hourly rate--Standard American English calls for the use of
an. While there are some exceptions
such as in the phrase, a unique experience, the noun that the
indefinite article modifies begins with a cosonant or a cosonant sound, Standard
American English calls for the use of a--e.g. a
glorious sunset.