A and An are called indefinite articles - they are used when referring to something the first time. An is used before vowels (and sometimes 'h' depending on pronunciation)
Examples would be:
"I am reading a book."
or
"I am reading an encyclopaedia."
--
The is a definite article and is used to refer to something specific. So:
"I am reading the book with the red cover."
or
"I am reading the second 'Lord of the Rings' book."
Hope that helps, although I think Isabel did that already.