Hi,
Exception into clause is used while changing the constraints status to enable validate with duplicate data in the table.After this clause is added all the duplicate data will go in this exception table.

Eg.

Create this table

create table exceptions(row_id rowid,
owner varchar2(30),
table_name varchar2(30),
constraint varchar2(30));

ALTER TABLE emp
ENABLE VALIDATE CONSTRAINT pk_emp
EXCEPTIONS INTO exceptions;

Now join this table row_id column with emp tables rowid to get the details of the records.

Hope it helps,
Take Care,
Nishant.