for fast retrieval of data from database we need index..
2 types of index r der-
1-primary index
2-secondary index
secondary index is created by default..

index is used to speed up the selectin of data from tables

secondary index is created in addition to the primary index 
of table

By using Index you can search a table for data records 
which will satisfy certain search criteria .Using index 
will speed up the search .An index can be considered a copy 
of the DB table that has been reduced to certain fields.
This copy is always in sorted form. Sorting provides faster 
access of the data records of the table.

There are 2 types :

  1) Primary index
  2) Secondary index

Shankar,
Only primary indexes are created by Default.
secondary indexes are created by the user, it can be 
created in SE11.