Index Work

Indexes are created for better performance of the DB ; for faster retrival of data from DB.
There are diffierent types of indexes...
(1) B tree index
(2) Bitmap map index
(3) Function based index
(4) Key compressed Index
(5) Unique index

For more information on Indexes Pls refer :http://docs.oracle.com/cd/B14117_01/server.101/b10759/statements_5010.htm

All of the indexes work in a different fashion since they are different from each other.

You can know whether the indexes are used or not by DB statistics and Execution Plan.

Best Regards,
Ash