The terminal 'Kill' command cancels the process which you choose to 'kill' and if it is a system process such as the Dock or Finder, it will relaunch them. To use it simply type - 'kill {Process ID}'. ( Replace Process ID with the actual process ID ) To see the process ID's of all running applications, open Activity Monitor.
The "killall" command can also be used to terminate a number of running processes. For example "killall Dock" kills the dock (reloads it)