You might need to stop (kill) a process that is in an endless loop, or stop a large job before it is completed. You can kill any process that you own. The system administrator can kill any process in the system except for those processes with process IDs of 0, 1, 2, 3, and 4. Killing these processes most likely will crash the system.
For more information, see the pgrep(1), pkill(1), and kill(1) man pages.