Killing a session in Oracle

Sessions in oracle can be killed with the command:

SQL> alter system kill session 'sid,serial#';

The value for sid and serial# can be query'd using the following query:

select * from v$session
or
select sid, serial#, osuser, program
from v$session;

After this command, the indicated session is marked for kill. When it's possible the session will be killed. Sometimes this can take a while (for example when a lot of rollback is needed).
To kill a session faster you can use the keyword IMMEDIATE like this:

SQL> alter system kill session 'sid,serial#' immediate;