Sunil,
For Oracle 10
g Release 2, refer to the Utilities volume of the documentation, which can be accessed from here:
http://www.oracle.com/pls/db102/portal.portal_db?selected=3 (You need to be a registered member of
OTN, but registration is free.)
Look for Part III - External Tables
Alternatively, you could search the
Ask Tom Web site for "external table".
Good Luck,
Avi.