Explanation: The kill command sends a defined signal to a process identified by a (PID):
$ kill - <signal> <pid>
Of course, we can only kill processes that we own, whereas root can kill all processes.

The signals that “kill” can send are listed in the man page (man kill). The signal can be represented by its name or by its PID. If no signal is specified, the default signal is 15 resp. So the “TERM” signal is used.

TERM (15): this will terminate a process, i.e. it executes a shutdown routine correctly.

KILL (9): applications are stopped and killed immediately (which could result in data loss)