The statement "select * from table_name where ... for update;" will hold rows of data for next DML statements in the same session. The selected rows is locked until a "commit" or a "rollback" command is issued. Any other session (even only a "select" statement) accessing those rows will have to wait until the lock is released. The Select For Update is easy to create a deadlock in the database. The lock can be seen from views:
SQL> select * from v$locked_object;
SQL> select * from v$transaction;
In a normal select, REDO is not used. But, the Select For Update will use REDO space.
SQL> set autotrace traceonly statistics
SQL> select * from test4update for update;
412167 rows selected.
Statistics
------------------------------------------------------
288 recursive calls
419550 db block gets
6003 consistent gets
0 physical reads
85955168 redo size 31676930 bytes sent via SQL*Net to client
192619 bytes received via SQL*Net from client
27479 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
412167 rows processed
