suppose u have one table which is in database. The table
contains some records. U want to modify one record of table.
      after modification, u don't want that new values for
the record. just u use rollback command. previous values are
placed in database. 
      
      Example:
      Case 1: Roll Back
      sno	name		palce
      1		abc		hyderabad
      2		xyz		bangalore.

      This is the table and it contains some records.
      Suppose if u want change the 2nd record, means palce
is changed to chennai.

      After modification:
      sno	name		palce
      1		abc		hyderabad
      2		xyz		Chennai.

      Suppose if u want old values, means u don't want new
values. At that time u use roll back command

      After useing rollback:
      sno	name		palce
      1		abc		hyderabad
      2		xyz		bangalore.
      after using roll back, the records are comes like
above(original records)

     Case 2: Commit
      sno	name		palce
      1		abc		hyderabad
      2		xyz		bangalore.

      This is the table and it contains some records.
      Suppose if u want change the 2nd record, means palce
is changed to chennai.

            After modification:
      sno	name		palce
      1		abc		hyderabad
      2		xyz		Chennai.

      After useing commit:
      sno	name		palce
      1		abc		hyderabad
      2		xyz		Chennai.
      after using commit, the table is finilased.u can't get
old values by uisng roll back.
      
Once if u use commit,u can't get old values for using of
rollback. Before commit, u can get old values for rollback.